Extend acceptable httplib2 version range.

A Beam Python 3 user reported an inconvenience when migrating their Python 2 to Python 3 pipeline due to a bug in httlib2 dependency, where httplib2 on Python3 doesn’t respect NO_PROXY environment variable. This bug was fixed in 0.13.1 [1]. Looking at the changelog of httplib2[2], there were more Python3-specific fixes in recent versions.

In the past we restricted httplib2 version due to a conflict with googledatastore[3]. We have since then removed[4] a dependency on googledatastore, and I don't see other reasons to restrict httplib2 to 0.12.0.

[1] https://github.com/httplib2/httplib2/pull/140
[2] https://github.com/httplib2/httplib2/blob/master/CHANGELOG
[2] https://github.com/apache/beam/commit/3b2c90156ddb67f4daddf275172c0b2d4eb1eaf6
[3] https://github.com/apache/beam/pull/11175/files#diff-e9d0ab71f74dc10309a29b697ee99330L202