Store local paths as pathlib paths

[EpicWink]

Instead of always having the file cache return strings, instead return pathlib.Path instances for local paths, and strings for URLs. This turns the string parsing for URL scheme into simply an isinstance check.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.43%. Comparing base (69a1d08) to head (cb14eb6).
⚠️ Report is 48 commits behind head on master.

Files with missing lines	Patch %	Lines
src/proxpi/_cache.py	64.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   89.33%   89.43%   +0.09%     
==========================================
  Files           3        3              
  Lines         741      729      -12     
==========================================
- Hits          662      652      -10     
+ Misses         79       77       -2     

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[EpicWink]

I need to perform benchmarking to see if this increases or reduces latency and server load