Test Python 3.8 and remove Python 3.6

[MaxGhenis]

Technical changes

• Test Python 3.8 (fixes Test Python 3.8 #1035)
• Remove Python 3.6 support in setup.py (fixes Remove Python 3.6 tags #1110)

Creating as a draft as I'm not sure I've done the Python version matrix correctly. I'm guessing some of the jobs should not be matrixed. I also need to adjust my linter.

NB: Python 3.8.12 is the latest version of Python 3.8, per https://www.python.org/downloads.

[coveralls]

Coverage increased (+0.02%) to 78.936% when pulling d91ecb8 on MaxGhenis:MaxGhenis/issue1035 into 49d95a0 on openfisca:master.

[MaxGhenis]

numpy 1.17.0 is failing to install on Python 3.8. I don't understand why it's starting at numpy 1.19.0 to retrieve the most recent three versions, rather than 1.22.0, which is the latest numpy version and compatible with Python 3.8 (not 3.7).

@maukoquiroga I'd missed your #1031 when starting this. Could this serve as a simpler starting point to at least get 3.8 running?

[MattiSG]

Thanks @MaxGhenis for this suggestion, and sorry for not having been able to collaborate on it to face the stronger complexity than it initially looked 😔
Superseded by #1168, which will hopefully provide a scalable way to test Python versions in a matrix. Looking forward to progressively bring support for newer versions of Python!