Update and standardize Python versions

[mdickinson]

This PR updates the CI workflows (other than the PyPI upload workflow - see #1567 for that) to standardize the Python versions:

• Where the Python version matters (that is, primarily for testing), include Python 3.10 in the test matrix
• Where the Python version isn't important (docs and style checks), standardize on Python 3.8 as the development version to use; this matches the current target for EDM.

[mdickinson]

From actions/setup-python#249, it looks as though we may actually be able to use plain old 3.10 now. I'll let the CI for the above run to completion, then try updating.

[mdickinson]

That's exciting: we've got an actual test failure (segfault) on 3.10:

======================================================================
ERROR: test_create_editor (traits.tests.test_enum.TestGui)
----------------------------------------------------------------------
TypeError: 'PySide2.QtCore.Qt.AlignmentFlag' object cannot be interpreted as an integer

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traits/tests/test_enum.py", line 353, in test_create_editor
    with UITester().create_ui(obj):
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traitsui/testing/tester/ui_tester.py", line 65, in __init__
    self._registries.extend(get_default_registries())
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traitsui/testing/tester/_ui_tester_registry/default_registry.py", line 38, in get_default_registries
    module = importlib.import_module(
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traitsui/testing/tester/_ui_tester_registry/qt4/default_registry.py", line 12, in <module>
    from traitsui.testing.tester._ui_tester_registry.qt4._traitsui import (
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traitsui/testing/tester/_ui_tester_registry/qt4/_traitsui/boolean_editor.py", line 16, in <module>
    from traitsui.qt4.boolean_editor import ReadonlyEditor, SimpleEditor
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traitsui/qt4/boolean_editor.py", line 34, in <module>
    from .text_editor import SimpleEditor as TextEditor
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traitsui/qt4/text_editor.py", line 34, in <module>
    from .editor_factory import ReadonlyEditor as BaseReadonlyEditor
  File "/opt/hostedtoolcache/Python/3.10.0-rc.2/x64/lib/python3.10/site-packages/traitsui/qt4/editor_factory.py", line 92, in <module>
    class ReadonlyEditor(Editor):
SystemError: _PyEval_EvalFrameDefault returned a result with an exception set

This smells like a bug in ctraits.c. (It could also be a problem in Python 3.10.0, but that seems much less likely.) Either way, it deserves investigation before Traits 6.3 is released.

[rahulporuri]

LGTM with one small comment

[rahulporuri]

I guess we're running the full testsuite with all supported pythons - which is why we can just ignore 3.7 and 3.9 here?

[mdickinson]

Good point; we should update this too, I think. (To include all Python versions.)

Dismissing because of the CI failure on Python 3.10 rc

[mdickinson]

The python version of 3.10 was being interpreted as a (floating-point) number, equal to 3.1. The fix is to add quotes to make it clear it's a string. Since it's fundamentally wrong to treat versions as numbers rather than strings, I've added those quotes to all python versions.

[mdickinson]

Converting to draft while we work through the various failures.

[mdickinson]

@rahulporuri Okay, I think this is looking okay now. Apologies for the false starts.

[mdickinson]

Sigh. Spoke too soon. There's still a macOS failure.

[mdickinson]

There's still a macOS failure.

This has to do with a bad NumPy build on Python 3.10. It's not clear why NumPy is being built at all - that is, why we're not simply using the wheel. I'll investigate.

[mdickinson]

It's not clear why NumPy is being built at all

NumPy 1.22 supports Python 3.10, but isn't released yet. NumPy 1.21 has preliminary support and test wheels for Linux, but doesn't officially support 3.10. We should just exclude NumPy from the test requirements on Python 3.10 for now.

[mdickinson]

Now this is ready for review.

[rahulporuri]

LGTM