parallelize pytest with pytest-xdist

[gboeing]

This PR parallelizes pytest in the CI tests and bash script with the pytest-xdist package, to run the tests simultaneously across 3 parallel processes. It runs the two longest-runtime tests (test_features() and test_graph_from()) each in their own processes, and groups all the other tests into a third process.

In total, this reduces the tests execution time by over 50%.

Github actions runners offer 4 CPUs, but we should probably limit it to 3 processes to avoid rate-limiting from the Overpass API if we're hitting it from multiple processes simultaneously.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.44%. Comparing base (844dd79) to head (325ccde).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1283   +/-   ##
=======================================
  Coverage   98.44%   98.44%           
=======================================
  Files          24       24           
  Lines        2383     2383           
=======================================
  Hits         2346     2346           
  Misses         37       37           

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