Update supported versions of Python, Django, Flask and Sanic

[jwhitlock]

The supported and tested versions should be updated to be limited to currently supported releases.

Python

The current Python list:

python-dockerflow/tox.ini

Lines 13 to 17 in 5e30d69

	python =
	3.6: py36
	3.7: py37
	3.8: py38
	3.9: py39

Supported versions are at: https://www.python.org/downloads/

We should:

• Drop Python 3.6 - Reached end-of-life on 2021-12-23 PEP 494
• Add Python 3.10 - Released 2021-10-04, end of support in 2026-10 PEP 619

Django:

The current Django list:

python-dockerflow/tox.ini

Lines 30 to 33 in 5e30d69

	dj22: -ctests/constraints/django-2.2.txt
	dj30: -ctests/constraints/django-3.0.txt
	dj31: -ctests/constraints/django-3.1.txt
	dj40: -ctests/constraints/django-4.0.txt

Supported versions are at: https://www.djangoproject.com/download/

We should:

• Drop Django 2.2 - Extended support ended April 2022
• Drop Django 3.0 - Extended support ended April 2021
• Drop Django 3.1 - Extended support ended December 2021
• Add Django 3.2 LTS - Supported until April 2024

Flask

The current Flask list:

python-dockerflow/tox.ini

Lines 34 to 36 in 5e30d69

	fl011: -ctests/constraints/flask-0.11.txt
	fl012: -ctests/constraints/flask-0.12.txt
	fl10: -ctests/constraints/flask-1.0.txt

Flask doesn't appear to have a supported release policy, but a v0.12.5 release was made in 2020-02-10 to fix an issue, almost 2 years after 1.0 was released, and a long-running v0.12 branch still exists. At the same time, this repo didn't keep up with version 1.1.0. Flask developers should speak up!

Possible next steps:

• Drop Flask 0.11 support, not tracked upstream
• Fix Flask 0.12 support
• Fix Flask 1.0 support
• Add Flask 1.1 support
• Add Flask 2.0 support
• Add Flask 2.1 support

Sanic

The current Sanic list:

python-dockerflow/tox.ini

Lines 37 to 38 in 5e30d69

	s19: -ctests/constraints/sanic-19.txt
	s20: -ctests/constraints/sanic-20.txt

Sanic versioning is calendar based, so this represents 2019 and 2020. The final release of the year (like 21.12) is the LTS release, supported for 24 months: https://sanic.dev/en/org/policies.html#versioning

Next steps:

• Drop Sanic 19, no longer supported
• Add Sanic 21, 21.12 supported until December 2023
• Add Sanic 22, currently 22.6

[jwhitlock]

Supporting Sanic 21 was harder than expected, and I have no experience with the framework, so I moved that to a new issue #70.

[jwhitlock]

Fixed in PR #69, with Sanic upgrades tracked in issue #70