wrapper.py: Replacing load_module() with exec_module()

Fixed "DeprecationWarning: the load_module() method is deprecated and
slated for removal in Python 3.12; use exec_module() instead." in
wrapper.py. Additionally removed Python 2 code (imp.load_source()).

Test: tox
Signed-off-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Change-Id: Ib7cc19b1c545f6449e034c4b01b582cf6cf4b581
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/353237


Reviewed-by: Mike Frysinger <vapier@google.com>