Pin: Add flexparser upper bound

[ndevenish]

Checklist

• Used a static YAML file for the patch if possible (instructions).
• Only wrote code directly into generate_patch_json.py if absolutely necessary.
• Ran pre-commit run -a and ensured all files pass the linting checks.
• Ran python show_diff.py and posted the output as part of the PR.
• Modifications won't affect packages built in the future.

show_diff output:

================================================================================
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linux-armv7l
================================================================================
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win-32
================================================================================
================================================================================
linux-ppc64le
================================================================================
================================================================================
osx-arm64
================================================================================
================================================================================
linux-aarch64
================================================================================
================================================================================
noarch
noarch::pint-0.24.1-pyhd8ed1ab_0.conda
noarch::pint-0.24.1-pyhd8ed1ab_1.conda
noarch::pint-0.24.3-pyhd8ed1ab_0.conda
-    "flexparser >=0.3",
+    "flexparser >=0.3,<0.4",
================================================================================
================================================================================
win-64
================================================================================
================================================================================
osx-64
================================================================================
================================================================================
linux-64

See conda-forge/pint-feedstock#71 and upstream comments; flexparser=0.4 broke with older pint versions. I've only patched 0.24.* here because that's when the flexparser >=0.3 constraint was added.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.