* Add pytest-cov; Remove code references to pip-tools and refactor method _build_package_list
* Regenerate lock file.
* Add back required typing-extensions, not sure how it got dropped from locking previously.
* Adopt the setup.cfg for coverage config.
* Actually pin to the version of click we have vendored in.
* Do not adopt coverage for CI since it slows everything down. Fix py36 dependency pinning.
* Drop support for 3.6 in this PR.
* No need to exclude the vendor and patched directory from the coverage reprot.
flake8 would have caught the regression in #371 and output:
`pipenv/cli.py:257:19: F821 undefined name 'hashed_deps_path'`
Whilst in an ideal world there would be 100% test coverage, linters help
fill in the gaps, plus spot potential unused variable/imports cleanup.
I'm happy to disable more of the style/whitespace pep8 rules if
needed - the priority is having the pyflakes subset run in CI.
Christian Clauss