Fix Retry allowed methods for urllib 1.25
This is a follow-up to ae4a3a6. It seems urllib devs went with a
*really* aggressive deprecation schedule here - the kwarg
name they deprecated in 1.26 does not exist in 1.25, which is
still shipped in Fedora 34 (and probably many other stable
distros). If you want the code to work on both 1.25 and 1.26
without any deprecation warnings, you need to fudge it like
this. Inspired by
https://github.com/firebase/firebase-admin-python/pull/532 .
Signed-off-by: Adam Williamson <awilliam@redhat.com>