httplib2 is a direct dependency but is not added as one

Hi! :wave:

When upgrading the package for this project on Arch Linux I noticed that the dependency on httplib2 is not declared.
It is used here:

https://github.com/iterative/PyDrive2/blob/10a60fc91056a0f11e3dfc7ff55ec8dad6ddf7c6/pydrive2/auth.py#L4

The httplib2 dependency is pulled in via oauth2client, but it would be cleaner to not rely on this transitive dependency and instead declare it as part of the dependencies of this project:

https://github.com/iterative/PyDrive2/blob/10a60fc91056a0f11e3dfc7ff55ec8dad6ddf7c6/setup.py#L38-L43

cc @grawlinson

Start in setup.py at the dependency list referenced by the issue, and inspect pydrive2/auth.py to confirm the direct import. Verify the package metadata declares httplib2 independently of oauth2client, then check the generated dependency information or packaging tests to confirm it is included.

Vom Indexierungsmodell aus dem Issue-Text verfasst.