Loosen Python dependency restrictions: httplib2, oauth2client

The Beam Python SDK's current pinned dependencies create dependency conflict issues for my team at Twitter.

I'd like the following expansions of the Python SDK's dependency ranges:

• oauth2client>=2.0.1,<4 to at least oauth2client>=2.0.1,<=4.1.2
• httplib2>=0.8,<=0.12.0 to at least httplib2>=0.8,<=0.12.3

I understand, from pull request 8653 by altay, that the latter is blocked in turn by googledatastore.