pytest: Move to pygit2 1.6.1

In order to move to a newer distribution for CI, we need to update
pygit2.  Given that bullseye ships with libgit2-1.1.x we can move up to
1.6.1 for pygit2 as it is the newest release that also still uses
libgit2-1.1.x

Signed-off-by: Tom Rini <trini@konsulko.com>