Add sane config for pytest

[eriknw]

This makes e.g. pytest w/o arguments only run the tests in toolz/.

Also fixed a test (to clean up resources), caught by filterwarnings = ["error"].

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (5a7e078) to head (51ecd3a).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #610   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         1184      1178    -6     
=========================================
- Hits          1184      1178    -6     

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