Use pytest < 7 because of conflict with pytest-cases

[bhazelton]

Description

Pin pytest < 7. This is a temporary fix for pytest-cases not being compatible with pytest 7.

I also added pytest-xdist for the external tests because they are now a limiting factor. Turns out it didn't help much, so I can remove it or leave it 🤷‍♀️ .

Motivation and Context

The conflict is being tracked here: smarie/python-pytest-cases#251
When it's fixed, we should require a newer version of pytest-cases and remove this pytest pin.

Types of changes

• Build or continuous integration change

Checklist:

• I have read the contribution guide.
• My code follows the code style of this project.

Build or continuous integration change checklist:

• If required or optional dependencies have changed (including version numbers), I have updated the readme to reflect this.
• If this is a new CI setup, I have added the associated badge to the readme and to references/make_index.py (if appropriate).

[codecov]

Codecov Report

Merging #1138 (1391eaf) into main (1d3959b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1138   +/-   ##
=======================================
  Coverage   99.83%   99.83%           
=======================================
  Files          31       31           
  Lines       15387    15387           
=======================================
  Hits        15361    15361           
  Misses         26       26           

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[mkolopanis]

Works for me. Let's revisit this once that issue is fixed.