Improve build system

[adamjanovsky]

There are two caveats in the current build system:

• setup.py is favored instead of nowadays preferred setup.cfg. We can use pip-tools to read unpinned requirements directly from setup.cfg (which is the only dynamic stuff we currently use setup.py for
• Tests are run with pinned dependencies against source (not installed version of sec-certs). Since we forecast that our tool will be mainly used as a library (not application, we run it in reproducible environment) it would make sense to run tests against installed version. This is easily achieved by creating the top-level module sec-certs into un-importable directory src/ as nowadays adopted by majority of Python projects.

Some reading:

• https://setuptools.pypa.io/en/latest/userguide/package_discovery.html#src-layout
• Stance (or discussion) on src/ directory pypa/packaging.python.org#320
• https://hynek.me/articles/testing-packaging/