Fix Python 3.9 fastjsonschema compatibility

[sbryngelson]

Description

Constrain fastjsonschema to versions below 2.22 on Python 3.9 while leaving Python 3.10 and newer unconstrained.

fastjsonschema 2.22.0 introduced PEP 604 type annotations such as dict | bool, which require Python 3.10, but the release does not declare a corresponding Requires-Python constraint. As a result, MFC's Python 3.9 compatibility job installs 2.22.0 and fails while importing fastjsonschema.

Upstream issue: horejsek/python-fastjsonschema#211

Observed MFC failure: https://github.com/MFlowCode/MFC/actions/runs/30178357153/job/89730733375

Type of change (delete unused ones)

• Bug fix

Testing

• uv pip compile toolchain/pyproject.toml --python-version 3.9 resolves fastjsonschema==2.21.2.
• uv pip compile toolchain/pyproject.toml --python-version 3.10 resolves fastjsonschema==2.22.0.
• ./mfc.sh lint -- 350 tests and 4 subtests passed.
• Commit precheck -- all seven checks passed.

Checklist

Check these like this [x] to indicate which of the below applies.

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

See the developer guide for full coding standards.

GPU changes (expand if you modified src/simulation/)

• GPU results match CPU results
• Tested on NVIDIA GPU or AMD GPU

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[Copilot]

Pull request overview

This PR fixes MFC’s Python 3.9 toolchain compatibility by preventing fastjsonschema versions that use Python 3.10-only PEP 604 type syntax from being installed on Python 3.9, while keeping Python 3.10+ unconstrained.

Changes:

• Add environment-marked fastjsonschema requirements to pin <2.22 on Python <3.10.
• Keep fastjsonschema unpinned for Python >=3.10.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.49%. Comparing base (76f8f51) to head (e653dab).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1678   +/-   ##
=======================================
  Coverage   60.49%   60.49%           
=======================================
  Files          84       84           
  Lines       21189    21189           
  Branches     3134     3134           
=======================================
  Hits        12819    12819           
  Misses       6303     6303           
  Partials     2067     2067           

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