FIX: importlib_resources-related edge cases

[TTsangSC]

Closes #405.

Summary

This fixes edge/corner cases which occur when importlib.resources is superseded during runtime by importlib_resources:

• If importlib_resources >= 6, < 6.4, line_profiler fails to import because line_profiler.toml_config.ConfigSource.from_default() can no longer use the now-nonexistent importlib.resources.path().
• If importlib_resources is even older, line_profiler generates a DeprecationWarning on import because importlib_resources.path() was deprecated in favor of importlib_resources.files().

(Such cases may happen e.g. when another package in the user's environment tries to be "smart" and replaces importlib.resources with importlib_resources in both the importlib namespace and sys.modules, via either explicit overwrites or tempering with the import machinery. Granted, this is probably a bad idea and most definitely not our problem, but the very fact that the issue exists shows that there may be a non-negligible amount of code out there which does this.)

A test (tests/test_toml_config.py::test_backported_importlib_resources()) has also been added to check that line_profiler.toml_config operates smoothly for:

• Vanilla, standard-lib importlib.resources
• When it's replaced by the following importlib_resources versions:
  • Default (newest on PyPI)
  • >= 6, < 6.4
  • < 6

Caveats

• This is some additional 150+ LoC for an edge case which is outside of our control and strictly speaking purview, since we don't even explicitly use importlib_resources in the repo.
• The new test is SLOW (some 10+ seconds) because it has to install line_profiler in a fresh venv where all the shenanigans with importlib_resources can happen.

Feel free to close if it's deemed that this is too much and we can just WONTFIX the original issue.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (60e928f) to head (09d4307).
⚠️ Report is 21 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #406      +/-   ##
==========================================
+ Coverage   87.56%   90.28%   +2.71%     
==========================================
  Files          18       20       +2     
  Lines        1641     2080     +439     
  Branches      348      447      +99     
==========================================
+ Hits         1437     1878     +441     
- Misses        149      151       +2     
+ Partials       55       51       -4     

Files with missing lines	Coverage Δ
line_profiler/toml_config.py	93.79% <100.00%> (ø)

... and 4 files with indirect coverage changes

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