pathlib, improve typings

[tony]

Fixes #278

• Use pathlib for system file paths
• Downloading: Improve typings of urlretrievefn and reporthook

[codecov]

Codecov Report

Merging #277 (28104f2) into master (272f8a4) will increase coverage by 0.02%.
The diff coverage is 89.36%.

❗ Current head 28104f2 differs from pull request most recent head d9798b4. Consider uploading reports for the commit d9798b4 to get more accurate results

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
+ Coverage   93.28%   93.30%   +0.02%     
==========================================
  Files          13       14       +1     
  Lines        1012     1045      +33     
  Branches      172      175       +3     
==========================================
+ Hits          944      975      +31     
- Misses         49       51       +2     
  Partials       19       19              

Impacted Files	Coverage Δ
src/unihan_etl/process.py	85.58% <85.41%> (+0.19%)	⬆️
src/unihan_etl/types.py	93.54% <86.66%> (-6.46%)	⬇️
tests/test_unihan.py	96.49% <90.00%> (-0.03%)	⬇️
src/unihan_etl/app_dirs.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more