Python 3 compatibility

[sffc]

When attempting to run corpuscrawler in Python 3, I get the error:

ImportError: No module named 'mimetools'

It would be good to support Python 3.

When running in Python 2, I get this other error:

AttributeError: 'module' object has no attribute 'SSLContext'

which apparently happens because that function was backported very recently to Python 2.

[brawer]

Want to make a pull request to fix it?