feat: Add Pixi workspace to pyproject.toml

[matthewfeickert]

Description

• Add Pixi workspace to pyproject.toml under [tool.pixi] table.
• Add Pixi lock file to .gitignore.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add Pixi workspace to pyproject.toml under [tool.pixi] table.
* Add Pixi lock file to .gitignore.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.23%. Comparing base (6b690c5) to head (a560690).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2638   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files          65       65           
  Lines        4190     4190           
  Branches      453      453           
=======================================
  Hits         4116     4116           
  Misses         45       45           
  Partials       29       29           

Flag	Coverage Δ
contrib	98.11% <ø> (ø)
doctest	98.23% <ø> (ø)
unittests-3.10	96.42% <ø> (ø)
unittests-3.11	96.42% <ø> (ø)
unittests-3.12	96.42% <ø> (ø)
unittests-3.13	96.42% <ø> (ø)
unittests-3.9	96.46% <ø> (ø)

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[matthewfeickert]

I've been making heavy use of https://pixi.sh/latest/python/pyproject_toml/#optional-dependencies for this PR and for testing.

I'm also realizing from this PR (/ remembering Issue #2622) that click-completion is in maintenance mode

pyhf/pyproject.toml

Line 67 in 6b690c5

shellcomplete = ["click_completion"]

and that there is a native Click v8 completion. c.f. https://click.palletsprojects.com/en/stable/shell-completion/#shell-completion