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

Attention: Patch coverage is 73.91304% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 89.56%. Comparing base (5afcd9e) to head (e57721e).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/proxpi/_cache.py	71.42%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   89.45%   89.56%   +0.11%     
==========================================
  Files           3        3              
  Lines         673      661      -12     
==========================================
- Hits          602      592      -10     
+ Misses         71       69       -2     

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[EpicWink]

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