Replace deprecated classifier with licence expression (PEP 639)

[hugovk]

Re:

• https://hugovk.dev/blog/2025/improving-licence-metadata/
• Replace deprecated classifier with licence expression (PEP 639) python-humanize/humanize#236

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.63%. Comparing base (5563e56) to head (f99a859).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #473   +/-   ##
=======================================
  Coverage   99.63%   99.63%           
=======================================
  Files           8        8           
  Lines         819      819           
=======================================
  Hits          816      816           
  Misses          3        3           

Flag	Coverage Δ
macos-latest	97.31% <ø> (ø)
ubuntu-latest	99.63% <ø> (ø)
windows-latest	97.31% <ø> (ø)

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