Remove Python 3.6 support; add Python 3.10 support

[thibaudcolas]

Description

This updates our test suite and the project’s metadata. Python 3.6’s support ends in 3 days. Python 3.10 has been out for a few months now and we need this package to support it.

The main difficulty here was getting Poetry to cooperate – they had a lot of Python 3.10 compatibility issues, and the JSONDecodeError fix from the latest v1.1.12 was required.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

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Suggested CHANGELOG entry:

### Added

- This package now supports Python 3.10 ([#163](https://github.com/torchbox/django-pattern-library/pull/163).

### Removed

- We no longer support Python 3.6, as it has reached its end of life ([#163](https://github.com/torchbox/django-pattern-library/pull/163).

[thibaudcolas]

Two things worth mentioning here:

• I thought it would be enough to only test djmain with the most recent Python version
• Similarly since py31-dj32 is already tested in the previous test job, I’ve removed it here.