Bump version to 2.9-git

CHANGELOG.md

-# Master (will become release 2.8)
+# Master (will become release 2.9)
+
+# Release 2.8
 
 - Python bindings have been moved from the `dune-python` module which is now
   obsolete. To activate Python bindings the CMake flag

dune.module

 Module: dune-geometry
-Version: 2.8-git
+Version: 2.9-git
 Author: The Dune Core developers
 Maintainer: dune-devel@lists.dune-project.org
 Description: Geometry Transformations, Reference Elements and Quadrature Rules
 URL: https://gitlab.dune-project.org/core/dune-geometry
-Depends: dune-common (>= 2.8)
+Depends: dune-common (>= 2.9)
 Whitespace-Hook: Yes