Fix for pip 19.3.1

[mgarnica]

Fixes #94
import main function from pip._internal.main for pip 19.3.1

[tikank]

Just a thought, that as pip user guide suggests to use pip program and the pip program is defined in entry points in setup.py, would getting that entry point to be a version independent way to find main function. Could something like this work:

Suggested change

	from pip._internal.main import main as pip_main
	pip_main = pkg_resources.load_entry_point('pip', 'console_scripts', 'pip')

[jayvdb]

fwiw, I've tested this PR on openSUSE Tumbleweed, and it fixed my builds.

Created #100 for a longer-term solution.