Update from Python 3.7 to 3.9

[jwhitlock]

Update to Python 3.9.10 in the Dockerfile and Heroku. This PR fixes #1591

How to test:

• Use pyenv or similar to install Python 3.9.10, create a new virtual environment, and install the requirements
• Run tests in developer mode: python3.9 -X dev -m pytest
• Run the server in developer mode, and poke around: python3.9 -X dev -m ./manage.py runserver

There was a warning about unclosed files in GetAttachmentTests:

ResourceWarning: unclosed file <_io.BufferedRandom name=14>

This is fixed by closing the temporary attachment stream during test teardown.

This leaves a few warnings that can be ignored for now:

pytest.yield_fixture warning

During test setup:

.../site-packages/pytest_django/fixtures.py:356: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
Use @pytest.fixture instead; they are the same.

This should be fixed by upgrading to pytest-django 4.0.0 or later, which support Python 3.9 and drops support for pytest before 5.4

yaml.dump warning

Collected during test run:

api/tests/views_tests.py::test_swagger_format[yaml]
  .../site-packages/drf_yasg/codecs.py:202: PendingDeprecationWarning: 
  dump will be removed, use
  
    yaml=YAML(typ='unsafe', pure=True)
    yaml.dump(...)
  
  instead
    return yaml.dump(data, Dumper=SaneYamlDumper, default_flow_style=False, encoding='utf-8' if binary else None)

api/tests/views_tests.py::test_swagger_format[yaml]
  .../site-packages/ruamel/yaml/main.py:1364: PendingDeprecationWarning: 
  dump_all will be removed, use
  
    yaml=YAML(typ='unsafe', pure=True)
    yaml.dump_all(...)
  
  instead
    return dump_all(

This was filed upstream (axnsan12/drf-yasg#765), but there has been no response in two months.

Unclosed .env warning

When starting the development server:

.../site-packages/env_tools.py:48: ResourceWarning: unclosed file <_io.TextIOWrapper name='.env' mode='r' encoding='UTF-8'>
  env = '[root]\n' + io.open(filename, 'r').read()
ResourceWarning: Enable tracemalloc to get the object allocation traceback

This is included by django-gulp. There's no issue filed, but also the projects haven't been update for years...

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[jwhitlock]

Rebased on main

[groovecoder]

Code looks good and spot-checks work: tests passed, and I spot-checked sending and receiving emails, replies, and with attachments too.

[jwhitlock]

@Vinnl - this turned out to be fairly easy, but it still may cause conflicts or merge issues with the React changes, so please review and merge on your schedule.

[Vinnl]

@jwhitlock React changes have already been merged in, and I don't see anything in here that should conflict with it, so feel free to merge! (And after the migration we can move away from Gulp, so that should resolve the django-gulp error as well.)