httplib2 is a direct dependency but is not added as one

[dvzrv]

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