httplib2 is a direct dependency but is not added as one

[dvzrv]

Hi! 👋

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

PyDrive2/pydrive2/auth.py

Line 4 in 10a60fc

import httplib2

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:

PyDrive2/setup.py

Lines 38 to 43 in 10a60fc

	install_requires=[
	"google-api-python-client >= 1.12.5",
	"oauth2client >= 4.0.0",
	"PyYAML >= 3.0",
	"pyOpenSSL >= 19.1.0",
	],

cc @grawlinson