[cmake][python] use `pip --upgrade` on install

This patch will always upgrade the Python package upon `make install`.

Previously, the Python package was only installed, if the package had not
been installed before. This violates the principle of least surprise as we
intuitively except `make install` to overwrite all files during the install
procedure.

cmake/modules/DunePythonInstallPackage.cmake

@@ -84,7 +84,7 @@ function(dune_python_install_package)
   endif()
 
   set(INSTALL_CMDLINE -m pip install
-                      "${INSTALL_OPTION}" "${WHEEL_OPTION}" "${EDIT_OPTION}" ${PYINST_ADDITIONAL_PIP_PARAMS}
+                      "${INSTALL_OPTION}" --upgrade "${WHEEL_OPTION}" "${EDIT_OPTION}" ${PYINST_ADDITIONAL_PIP_PARAMS}
                       "${PYINST_FULLPATH}")