Test release

Add a MANIFEST.in file

This is solely to workaround setuptool's weird behaviour of not
exercising its default package discovery logic when the setuptools-scm
plugin had to fall back to a static version string during its version
inference.
Due to setuptools-scm order of precedence for git metadata discovery,
this file should only ever be used on a rare occasion, e.g. doing
development builds from a git worktree.

[1] https://setuptools-scm.readthedocs.io/en/latest/usage/#file-finders-hook-makes-most-of-manifestin-unnecessary

Signed-off-by: Erik Skultety <eskultet@redhat.com>