Parse version number out of requests/__init__.py

Parse version number out of requests/__init__.py

With the new import machinery in requests/packages/__init__.py people
using `python setup.py test` will not be able to run the tests the first
time on a new environment. The VendorAlias logic breaks the import
machinery and the tests fail. By removing the import of requests in
setup.py, we avoid the issue where sys.meta_path continues to have the
version of VendorAlias from the import in setup.py.

Closes #2462