Pin flexparser before it breaks pint

[mraspaud]

See hgrecco/pint#1969

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.10%. Comparing base (a0a7f53) to head (9d6a879).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2971   +/-   ##
=======================================
  Coverage   96.10%   96.10%           
=======================================
  Files         377      377           
  Lines       55134    55134           
=======================================
  Hits        52984    52984           
  Misses       2150     2150           

Flag	Coverage Δ
behaviourtests	3.94% <ø> (ø)
unittests	96.19% <ø> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 11719620398

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.206%

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Totals
Change from base Build 11692501028:	0.0%
Covered Lines:	53228
Relevant Lines:	55327

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💛 - Coveralls