Support importing from Wheels automatically
Drop Python 3.3 for unbundled

We're targeting these changes at downstream re-distributors and it's
unlikely that one of them will be willing to have a new version of
pip without the most recent version of Python 3.
Drop PyPy from the de-bundling tests

PyPy isn't often packages by downstream, and even when it it is
it's often not got a pip package to go with it, and if it does
then it's probably not very likely that this code will fail on
PyPy but not on CPython 2.7.

So we'll just drop them so that the tests complete faster.
Merge pull request #2184 from dstufft/support-import-wheels

Add Wheels to sys.path If They Exist In The Vendor Directory