fix(python): Refine interaction of "schema_overrides" with read_excel when using "calamine" engine

[alexander-beedie]

Fixes a potential edge-case where values that appear to be all-integer (but are explicitly designated as float in "schema_overrides") are converted back to integer instead of respecting the override.

Added suitable/new test coverage.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.38%. Comparing base (a078d0c) to head (f44dc76).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15827   +/-   ##
=======================================
  Coverage   80.38%   80.38%           
=======================================
  Files        1263     1263           
  Lines      165351   165368   +17     
=======================================
+ Hits       132910   132932   +22     
+ Misses      32441    32436    -5     

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