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 32.71%. Comparing base (dc506f6) to head (1c2c1a4).
Report is 3 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (dc506f6) and HEAD (1c2c1a4). Click for more details.

HEAD has 18 uploads less than BASE

Flag	BASE (dc506f6)	HEAD (1c2c1a4)
	23	5

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1831       +/-   ##
===========================================
- Coverage   92.29%   32.71%   -59.58%     
===========================================
  Files          62       62               
  Lines        7471     7471               
===========================================
- Hits         6895     2444     -4451     
- Misses        576     5027     +4451     

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