Clarify imports that are dependent on the version of Python

[rmartin16]

Changes

• For imports that are dependent on the version of Python, make that explicit and use a syntax that pyupgrade can automatically handle for Python EOLs
• Given that pyupgrade can change imports, run it before isort runs in pre-commit
• Installing toml package is no longer necessary to run isort in pre-commit
• Instead of assuming knowledge about codespell's usage of TOML (such as the fact it uses the tomli package), instruct pre-commit to install the toml extra for codespell

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

Changing the default skin for the Android emulator broke the testbed tests:

I/python.stdout: =================================== FAILURES ===================================
I/python.stdout: _________________________________ test_scroll __________________________________
I/python.stdout: Traceback (most recent call last):
I/python.stdout:   File "/data/data/org.beeware.toga.testbed/files/chaquopy/AssetFinder/app/tests/widgets/test_detailedlist.py", line 107, in test_scroll
I/python.stdout:     assert probe.scroll_position == pytest.approx(
I/python.stdout: AssertionError: assert 4000.0 == 3469.5238095238096 ± 4.0e+02
I/python.stdout:   comparison failed
I/python.stdout:   Obtained: 4000.0
I/python.stdout:   Expected: 3469.5238095238096 ± 4.0e+02
I/python.stdout: _________________________________ test_scroll __________________________________
I/python.stdout: Traceback (most recent call last):
I/python.stdout:   File "/data/data/org.beeware.toga.testbed/files/chaquopy/AssetFinder/app/tests/widgets/test_table.py", line 126, in test_scroll
I/python.stdout:     assert probe.max_scroll_position > probe.height * 2
I/python.stdout: AssertionError: assert 1286.095238095238 > (1061 * 2)
I/python.stdout:  +  where 1286.095238095238 = <tests_backend.widgets.table.TableProbe object at 0x70811b283d60>.max_scroll_position
I/python.stdout:  +  and   1061 = <tests_backend.widgets.table.TableProbe object at 0x70811b283d60>.height
I/python.stdout: =========================== short test summary info ============================
I/python.stdout: FAILED tests/widgets/test_detailedlist.py::test_scroll - assert 4000.0 == 346...
I/python.stdout: FAILED tests/widgets/test_table.py::test_scroll - assert 1286.095238095238 > ...
I/python.stdout: ====== 2 failed, 328 passed, 61 skipped, 25 xfailed in 195.32s (0:03:15) =======

[rmartin16]

I was able to resolve the testbed failures by having the AVD use pixel_3a skin again.

[freakboy3742]

At some point, we'll probably need to update the android tests to reflect changes in device skin; but otherwise, this all makes sense.