Python 3.9 compatibility (resolve DeprecationWarning from collections)

Issue Summary

Wagtail hits the warning DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working in various places.

• Jinja2>2.10.1
• html5lib>1.0.1 (see https://github.com/html5lib/html5lib-python/issues/419)
• beautifulsoup4>=4.8.0 (or maybe earlier)
• fix wagtail/utils/l18n/translation.py:5 (#5485)

Steps to Reproduce

1. run tox -e py37-dj22-sqlite-noelasticsearch -- --deprecation all
2. note deprecation warnings:

site-packages/jinja2/utils.py:485: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
site-packages/jinja2/runtime.py:318: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

resolved by https://github.com/pallets/jinja/pull/867 - the fix is on master not released yet (as of Jinja2==2.10.1)

site-packages/html5lib/_trie/_base.py:3: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Mapping

Fixed in https://github.com/html5lib/html5lib-python/issues/402 , but not released yet as of 1.0.1 (see https://github.com/html5lib/html5lib-python/issues/419 )

site-packages/bs4/element.py:1134: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  if not isinstance(formatter, collections.Callable):

https://bugs.launchpad.net/beautifulsoup/+bug/1778909 - resolved by beautifulsoup4>=4.8.0 (and earlier I think)

I’m also seeing one in Wagtail from my own project tests here, the above tox run didn’t seem to hit it:

wagtail/utils/l18n/translation.py:5: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working

See #5485

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project:

yes (on master at bb4e2fe2dfe69fc3143f38c6e34dbe6f2f2f01e0 )

Technical details

• Python version: Run python --version.

Python 3.7.3

• Django version: Look in your requirements.txt, or run pip show django | grep Version.

Django 2.2

• Wagtail version: Look at the bottom of the Settings menu in the Wagtail admin, or run pip show wagtail | grep Version:.

2.7.0.alpha.0

[gasman]

@loicteixeira I think it’s a good thing to keep our dependency versions as wide as possible - the fact that we offer Python 3.8 support doesn’t mean that we should lock down our dependencies to only the ones that are 3.8-compatible. At the extreme end, that means that we’re restricting people to a version of Pillow that’s only a few days old.

On one hand, it does mean that early adopters of Python 3.8 may have to figure out the required upgrades for themselves (hopefully nothing that a pip install -U can’t solve); on the other hand, it means that people who are stuck on older versions of libraries for whatever reason (for example: they’re running on a platform like Google AppEngine with limited ability to install server-level packages) aren’t being left stranded.

[gasman]

Apparently the collections.abc change was deferred again to Python 3.10, but in any case, the final remaining deprecation warning was in botocore (a testing-only dependency), and version-bumping that has resolved it. Will open a PR shortly to make Python 3.9 support official (and current indications are that we’ll be able to support 3.10 with no further changes too).