tests: use modern tmp_path instead of py.path based tmpdir

[henryiii]

This uses pathlib-based tmp_path instead of py.path based tmpdir fixtures. Should be fine for pytest 6+, I think. Came up in the context of #512, though you can't quite remove the _pytest warning suppression, as there's one remaining thing in pytest that is missing an encoding.

[agronholm]

I actually made this change in the publicapi branch. I expected that work to proceed faster than it actually has 😃

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (0de0c1a) 68.35% compared to head (1b69248) 68.35%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #513   +/-   ##
=======================================
  Coverage   68.35%   68.35%           
=======================================
  Files          12       12           
  Lines         929      929           
=======================================
  Hits          635      635           
  Misses        294      294           

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