Use setuptools extras_require to install optional dependencies

PyMongo has a few optional dependencies for certain situations:

• GSSAPI auth requires pykerberos on *nix or (soon) winkerberos on Windows.
• TLS is far simpler to use with old python versions (<= 2.7.9) if certifi or wincertstore is installed.

Use extras_require to optionally depend on the right packages on the right platforms.

https://pythonhosted.org/setuptools/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies
https://pip.pypa.io/en/latest/reference/pip_install/#examples

After these changes you should be able to do the following:

python -m pip install pymongo[gssapi]
python -m pip install pymongo[tls]
python -m pip install pymongo[gssapi, tls]

If the python version is new enough to not need certifi or wincertstore pymongo[tls] shouldn't install either of them.