setup.py improvements

[nbraud]

Mostly the same thing as ppb/pursuedpybear#223 and ppb/pursuedpybear#224 :

• Rename *-requirements.txt to requirements-*.txt to follow PPB's convention.
• Extract runtime dependencies to requirements.txt.
• Move all metadata from setup.py to setup.cfg.
• Declare test dependencies in setup.py, integrate with pytest.
• Use pyproject.toml to explicitly declare that we are using setuptools.

[nbraud]

I ran into a Travis issue, which I fixed in the latest last 2 commits here.

I'd honestly prefer to remove that commit and do away with Travis (by merging #127):
It's been almost 2 weeks and 13 pull requests since we set up Cirrus, and while the situation was different earlier (because Cirrus was newer and we didn't need to touch Travis for it to keep working), I'm not keen on maintaining 2 parallel CIs.

Update: Travis was dropped, Travis-related changes were dropped, see below.

[nbraud]

OK, this is ready for review and merge; I removed the Travis fixes that would have conflicted with #127

[nbraud]

PS: I chose not to make setup.py test run the linters for two reasons:

0. A new version of the linter may add new warnings, but that's no reason to fail setup.py test && setup.py install, especially since most Linux distros use something equivalent when packaging Python things.
1. It would make the linters a test dependency (hence, a build dependency in contexts where packages are tested at build time).

[AstraLuma]

lgtm
bors r+

[bors]

Build succeeded

• docs
• FreeBSD PYTHON:3.6
• FreeBSD PYTHON:3.7
• lint
• Linux container:python:3.6-slim
• Linux container:python:3.7-slim
• macOS PYTHON:3.6.8
• macOS PYTHON:3.7.2
• Windows container:python:3.6-windowsservercore-1809
• Windows container:python:3.7-windowsservercore-1809