Actually, pygit2 supports auth in 0.20.3 as well

Allow for a minimum version of 0.20.3. The one difference found so far
is that pygit2.Remote.fetch() returns a class instance starting in
0.21.0, so this commit also changes the logic that checks for the number
of received objects so that it works irrespective of version.
Update docs to reflect pygit2 auth support in 0.20.3