pytest: Lower pygit2 requirement

The latest versions of pygit2 are not available in practically any
distribution at this time.  Furthermore, we don't need the latest in
order to run all of our testsuites.  Reduce this version requirement to
something older that meets our needs while still supporting running our
tests on older hosts (and so, test labs).

Reported-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Tom Rini <trini@konsulko.com>