MANIFEST: Fix file name extensions
The files got new file name extensions, but MANIFEST was not updated -> files were missing in the tarballs
-include AUTHORS.rst
-include CONTRIBUTING.rst
-include HISTORY.rst
+include AUTHORS.md
+include CONTRIBUTING.md
+include HISTORY.md
include LICENSE
-include README.rst
+include README.md
recursive-include tests *
recursive-exclude * __pycache__
