Reorganize changelog for Dune 2.7

Add subsections for deprecations/removals, build system.
Add removal of Python 2 support

CHANGELOG.md

@@ -2,20 +2,6 @@
 
 - Added fallback implementation to C++20 feature: `std::identity`.
 
-- Dune no longer applies architecture flags detected by the Vc library
-  automatically.  This applies to all targets that link to Vc explicitly (with
-  `add_dune_vc_flags()`) or implicitly (with `dune_enable_all_packages()`).
-  If you do want to make use of extended architecture features, set the
-  architecture explicitly in the compiler options, e.g. by specifying
-  ```sh
-  CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-march=native"
-  ```
-  in your opts-file.  Vc also sets compiler options to select a particular C++
-  abi (`-fabi-version` and `-fabi-compat-version`), these continue to be
-  applied automatically.
-
-  See core/dune-common!677
-
 - A helper class `TransformedRangeView` was added representing a
   transformed version of a given range using an unary transformation
   function. The transformation is done on the fly leaving the wrapped
@@ -52,53 +38,73 @@
   powers with an integer exponent, and is usable in compile-time expressions.
   The use of the old power methods in `power.hh` is henceforth discouraged.
 
--   When run with an absolute build directory, `dunecontrol` now exposes the root build
-    directory to CMake in the variable `DUNE_BUILD_DIRECTORY_ROOT_PATH`.
+-   `FieldMatrix` and `FieldVector` are now [trivially copyable types]
+    if the underlying field type is trivially copyable.
 
-    See core/dune-common!542
+    As a consequence the copy assignment operator of the `DenseVector`
+    class can no longer be used; just avoid going through
+    `DenseVector` and use the real vector type instead
+    (e.g. `FieldVector`).
 
--   LaTeX documents can now be built using `latexmk` with the help of UseLatexmk.cmake's
-    `add_latex_document`. `dune_add_latex_document` will use the new way of calling
-    LaTeX when the first argument is `SOURCE`. As a side effect, in-source builds are
-    supported, too. The old function call and UseLATEX.cmake are deprecated and will be
-    removed after 2.7.
+    [trivially copyable types]: https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable
 
-    See core/dune-common!594
+## Deprecations and removals
 
--   The build system has learned some new tricks when creating or looking for the Python virtualenv:
-    When using an absolute build directory with `dunecontrol`, the virtualenv will now be placed
-    directly inside the root of the build directory hierarchy in the directory `dune-python-env`.
-    This should make it much easier to actually find the virtualenv and also avoids some corner
-    cases where the build system would create multiple virtualenvs that did not know about each
-    other. This behavior can be disabled by setting
-    `DUNE_PYTHON_EXTERNAL_VIRTUALENV_FOR_ABSOLUTE_BUILDDIR=0`.
-    If you need even more precise control about the location of the virtualenv, you can now also
-    directly set the CMake variable `DUNE_PYTHON_VIRTUALENV_PATH` to the directory in which to
-    create the virtualenv.
+- The `VectorSize` helper has been deprecated.  The `size()` method of
+  vectors should be called directly instead.
 
--   The `dune_symlink_to_sources_files` cmake function now has a `DESTINATION` argument.
+- Drop support for Python 2. Only Python 3 works with Dune 2.7.
 
 - Support for older version than METIS 5.x and ParMETIS 4.x is deprecated and will be
   removed after Dune 2.7.
 
+## build-system
+
+- When run with an absolute build directory, `dunecontrol` now exposes the root build
+  directory to CMake in the variable `DUNE_BUILD_DIRECTORY_ROOT_PATH`.
+
+  See core/dune-common!542
+
+- The `dune_symlink_to_sources_files` CMake function now has a `DESTINATION` argument.
+
+- Dune no longer applies architecture flags detected by the Vc library
+  automatically.  This applies to all targets that link to Vc explicitly (with
+  `add_dune_vc_flags()`) or implicitly (with `dune_enable_all_packages()`).
+  If you do want to make use of extended architecture features, set the
+  architecture explicitly in the compiler options, e.g. by specifying
+  ```sh
+  CMAKE_FLAGS="-DCMAKE_CXX_FLAGS=-march=native"
+  ```
+  in your opts-file.  Vc also sets compiler options to select a particular C++
+  abi (`-fabi-version` and `-fabi-compat-version`), these continue to be
+  applied automatically.
+
+  See core/dune-common!677
+
 - `FindParMETIS.cmake` assumes METIS was found first using `FindMETIS.cmake` and does not
   longer try to find METIS itself.
 
 - The `inkscape_generate_png_from_svg` CMake function is deprecated and will be removed
   after 2.7.
 
-- The `VectorSize` helper has been deprecated.  The `size()` method of
-  vectors should be called directly instead.
-
--   `FieldMatrix` and `FieldVector` are now [trivially copyable types]
-    if the underlying field type is trivally copyable.
-
-    As a consequence the copy assignment operator of the `DenseVector`
-    class can no longer be used; just avoid going through
-    `DenseVector` and use the real vector type instead
-    (e.g. `FieldVector`).
-
-    [trivially copyable types]: https://en.cppreference.com/w/cpp/named_req/TriviallyCopyable
+- LaTeX documents can now be built using `latexmk` with the help of UseLatexmk.cmake's
+  `add_latex_document`. `dune_add_latex_document` will use the new way of calling
+  LaTeX when the first argument is `SOURCE`. As a side effect, in-source builds are
+  supported, too. The old function call and UseLATEX.cmake are deprecated and will be
+  removed after 2.7.
+
+  See core/dune-common!594
+
+- The build system has learned some new tricks when creating or looking for the Python virtualenv:
+  When using an absolute build directory with `dunecontrol`, the virtualenv will now be placed
+  directly inside the root of the build directory hierarchy in the directory `dune-python-env`.
+  This should make it much easier to actually find the virtualenv and also avoids some corner
+  cases where the build system would create multiple virtualenvs that did not know about each
+  other. This behavior can be disabled by setting
+  `DUNE_PYTHON_EXTERNAL_VIRTUALENV_FOR_ABSOLUTE_BUILDDIR=0`.
+  If you need even more precise control about the location of the virtualenv, you can now also
+  directly set the CMake variable `DUNE_PYTHON_VIRTUALENV_PATH` to the directory in which to
+  create the virtualenv.
 
 # Release 2.6