using pytest-xdist for tests

[williambdean]

Description

I saw this over in another python package (holidays) and would like to see if this works for ours...

Related Issue

• Closes #
• Related to #

Checklist

• Checked that the pre-commit linting/style checks pass. Feel free to comment pre-commit.ci autofix to auto-fix.
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks) using numpydoc format.
• If you are a pro: each commit corresponds to a relevant logical change

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📚 Documentation preview 📚: https://pymc-marketing--1831.org.readthedocs.build/en/1831/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.40%. Comparing base (63cdc8a) to head (0112db3).

❗ There is a different number of reports uploaded between BASE (63cdc8a) and HEAD (0112db3). Click for more details.

HEAD has 17 uploads less than BASE

Flag	BASE (63cdc8a)	HEAD (0112db3)
	21	4

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1831       +/-   ##
===========================================
- Coverage   91.89%   31.40%   -60.49%     
===========================================
  Files          64       64               
  Lines        7572     7572               
===========================================
- Hits         6958     2378     -4580     
- Misses        614     5194     +4580     

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