Unix: Fallback to default if XDG environment variable is empty

[papr]

Fixes #29 using https://github.com/pypa/pip/blob/867bbb00ccb0506095e19a0e111ce06ce71a2495/tools/vendoring/patches/appdirs.patch#L47-L54 as a reference implementation.

Open questions:

• How to test this properly?
• How to test compatibility with appdirs if appdirs is not implementing this behavior?

Todo:

• Tests
• Changelog

[codecov-commenter]

Codecov Report

Merging #30 (9543cb1) into main (f9c2074) will increase coverage by 3.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   90.70%   93.89%   +3.18%     
==========================================
  Files          12       13       +1     
  Lines         495      524      +29     
  Branches       44       44              
==========================================
+ Hits          449      492      +43     
+ Misses         32       26       -6     
+ Partials       14        6       -8     

Flag	Coverage Δ
tests	93.51% <100.00%> (+3.20%)	⬆️

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Impacted Files	Coverage Δ
src/platformdirs/unix.py	96.82% <100.00%> (+22.22%)	⬆️
tests/test_unix.py	100.00% <100.00%> (ø)

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[papr]

I will need help with the failing type checks. I am able to run tox -e type but it is raising more errors than logged in the Github Actions. Some of them affect files that I did not change here but in #27 where the type check ran successfully. What am I missing?

[gaborbernat]

I agree of inline-ing the new functionality, because we don't want to call the expesnive relatively os.path.expanduser unless we must. The tests just needed some type annotation, and you cannot use the Platformdir class directly as type because is dynamically calculated and mypy cannot deduce it. You'd need to type it as the ABC variant, but in this case we can just always run the tests on all platforms and annotate it as Unix, because the functionality you've added is tied to the Unix class, not anything else in the __init__.py file.