Drop compatiblity with Python 3.6

[nbraud]

We only aim to support the last 2 major versions of Python (currently 3.7 and 3.8), and the dataclasses backport for Python 3.6 is unmaintained and broken (see discussion in #184).

This should allow us to unblock #168

[nbraud]

I noticed a disadvantage while updating the CI config: PyPy3 recently had a stable release, but it still only supports Python 3.6.9, so we would be dropping PyPy support (until they move to Py3.7).

I don't think this is a major issue, as ppb itself doesn't support PyPy.

[AstraLuma]

Not at this time, but I'd be ok pursuing pypy compatibility for performance reasons? There's other complications, though.

If the dataclasses bugs cause problems, let's drop it from ppb-vector; we're only using it for utility reasons.

[nbraud]

If the dataclasses bugs cause problems

To be clear, that's not a bug in the stdlib's dataclasses module (at least, not as found in any version of Python ever released).

The issue is that the (unofficial) PyPI package is essentially a snapshot from an early development version of dataclasses from before it was first included in a stable Python release.

[AstraLuma]

bors r+

[bors]

Build failed

• Linux python:3.7-slim
• Linux python:3.8-slim
• macOS PYTHON:3.7.5
• macOS PYTHON:3.8.0
• Windows python:3.7-windowsservercore-1809

[nbraud]

bors r=astraluma

[bors]

Build failed:

• lint

[nbraud]

bors r=astraluma

[bors]

Build failed:

• FreeBSD PYTHON:3.9

[nbraud]

OK, I guess I'm making a CI-fixing PR first >_>'

[nbraud]

bors r=astraluma

[bors]

Build succeeded:

• docs
• FreeBSD PYTHON:3.7
• FreeBSD PYTHON:3.8
• FreeBSD PYTHON:3.9
• lint
• Linux python:3.7-slim
• Linux python:3.8-slim
• Linux python:3.9-slim
• macOS PYTHON:3.7.7
• macOS PYTHON:3.8.6
• macOS PYTHON:3.9.0
• pep517
• Windows python:3.7-windowsservercore
• Windows python:3.8-windowsservercore
• Windows python:3.9-windowsservercore