Defer imports for faster overall import time: 19 ms -> 5 ms

[hugovk]

Defer imports, especially avoid typing import for normal use, which is getting slower in 3.14, to make the overall import time of humanize faster.

Before: 19 ms

After: 5 ms

Also move mypy dependency to autoupdateable requirements file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.48%. Comparing base (c6455cf) to head (147cbbc).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #234      +/-   ##
==========================================
- Coverage   99.49%   99.48%   -0.01%     
==========================================
  Files          11       11              
  Lines         785      782       -3     
==========================================
- Hits          781      778       -3     
  Misses          4        4              

Flag	Coverage Δ
macos-latest	97.69% <100.00%> (-0.01%)	⬇️
ubuntu-latest	97.69% <100.00%> (-0.01%)	⬇️
windows-latest	95.90% <100.00%> (-0.02%)	⬇️

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