dehumanize: reject non-string input before locale matching

[HrachShah]

Arrow.dehumanize() documents a string input, but non-string values previously reached locale matching and could fail with implementation-dependent errors. Validate the public argument before iterating locale timeframes so callers consistently receive a TypeError naming the expected type.

Added a regression for None.

Tests: python -m pytest tests/test_arrow.py -k "TestArrowDehumanize and non_string_input" --override-ini="addopts="

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (09b7160).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1330   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2318    +3     
  Branches       358       359    +1     
=========================================
+ Hits          2315      2318    +3     

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