Bump version to 2.9-git

CHANGELOG.md

-# Master (will become release 2.8)
+# Master (will become release 2.9)
+
+# Release 2.8
 
 - Extended the MatrixMarket IO functions for reading and writing vectors with
   SIMD field_type as tall-skinny matrices.

dune.module

 Module: dune-istl
-Version: 2.8-git
+Version: 2.9-git
 Author: The Dune Core developers
 Maintainer: dune-devel@lists.dune-project.org
 Description: Iterative solver template library which provides generic sparse matrix/vector classes and a variety of solvers based on these classes
 URL: https://gitlab.dune-project.org/core/dune-istl
 Python-Requires:
-Depends: dune-common (>= 2.8)
+Depends: dune-common (>= 2.9)
 Whitespace-Hook: Yes