improve license detection

[garbas]

license should also be picked from classifiers

[seppeljordan]

This is the output if we try to run pypi2nix on its own requirements.

WARNING: Couldn't recognize license `UNKNOWN` for `setupcfg`
WARNING: Couldn't recognize license `UNKNOWN` for `orderdict`
WARNING: Couldn't recognize license `UNKNOWN` for `zipp`
WARNING: Couldn't recognize license `http://www.apache.org/licenses/LICENSE-2.0` for `codecov`
WARNING: Couldn't recognize license `PSF` for `typing-extensions`
WARNING: Couldn't recognize license `UNKNOWN` for `flit`
WARNING: Couldn't recognize license `MIT license` for `py`
WARNING: Couldn't recognize license `UNKNOWN` for `setuptools`
WARNING: Couldn't recognize license `BSD-3-Clause` for `MarkupSafe`
WARNING: Couldn't recognize license `Expat license` for `pycodestyle`
WARNING: Couldn't recognize license `UNKNOWN` for `write`
WARNING: Couldn't recognize license `MIT license` for `pytest`
WARNING: Couldn't recognize license `MIT license` for `pluggy`
WARNING: Couldn't recognize license `UNKNOWN` for `entrypoints`
WARNING: Couldn't recognize license `UNKNOWN` for `intreehooks`
WARNING: Couldn't recognize license `UNKNOWN` for `values`
WARNING: Couldn't recognize license `Apache Software License` for `importlib-metadata`
WARNING: Couldn't recognize license `UNKNOWN` for `mkdir`
WARNING: Couldn't recognize license `UNKNOWN` for `public`
WARNING: Couldn't recognize license `GPL version 2` for `flake8-isort`
WARNING: Couldn't recognize license `public domain, Python, 2-Clause BSD, GPL 3 (see COPYING.txt)` for `docutils`
WARNING: Couldn't recognize license `Expat license` for `mccabe`
WARNING: Couldn't recognize license `UNKNOWN` for `pytest-runner`
WARNING: Couldn't recognize license `GPLv3` for `nix-prefetch-github`

[seppeljordan]

Addressed by #353.