Make dnspython a required dependency

dnspython not being a required dependency causes no end of confusion for Atlas users. Let's make it PyMongo's first and only required dependency. If making it a required dep causes a lot of pain for users stuck behind corporate firewalls we can consider releasing a pymongo-core package for those users.

This should only be a small change in setup.py, moving dnspython from extras_require to install_requires, with some documentation updates. Let's leave the "srv" extra around so as to not break existing users.