build: remove pytest-runner (fixes #749)

[hydrosquall]

Summary of Changes

Fixes #749

I don't think we actually use pytest-runner to invoke our tests, so it's better just to remove this entirely if it's creating security warnings.

Checklist

• Code follows the style guidelines of this project
• Added tests for new functionality
• Update HISTORY.rst with an entry summarizing your change
• Tag a project maintainer

[hydrosquall]

Current dependencies on/for this PR:

• master
  • PR build: remove pytest-runner (fixes #749) #845 👈

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (199d3ac) 90.47% compared to head (0fd003a) 90.47%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #845   +/-   ##
=======================================
  Coverage   90.47%   90.47%           
=======================================
  Files           6        6           
  Lines         294      294           
=======================================
  Hits          266      266           
  Misses         28       28           

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