fix(factory): treat explicit tzinfo=None the same as omitting tzinfo

[theohmwoa]

Summary

arrow.get(value, format, tzinfo=None) raised TypeError: Arrow.__init__() missing 1 required positional argument: 'day' because the factory's routing check used the value of tz (which defaults to None) to detect whether the tzinfo kwarg was supplied. When a caller explicitly passed tzinfo=None, the check incorrectly forced arg_count = 3 and routed the call to the multi-arg Arrow(year, month, day, ...) constructor path.

The fix replaces tz is None with "tzinfo" not in kwargs, which actually answers the question the existing comment already promised (# tzinfo kwarg is not provided).

Reproduction

import arrow
arrow.get('2025-01-01', 'YYYY-MM-DD', tzinfo=None)
# Before: TypeError: Arrow.__init__() missing 1 required positional argument: 'day'
# After:  <Arrow [2025-01-01T00:00:00+00:00]>

Test

Added TestGet.test_two_args_str_str_tzinfo_none in tests/test_factory.py. It asserts that factory.get("2025-01-01", "YYYY-MM-DD", tzinfo=None) no longer raises, equals factory.get("2025-01-01", "YYYY-MM-DD"), and yields the expected UTC datetime. Full suite passes (1903 tests, 99.93% coverage).

Fixes #1259.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (6a379ef).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1277   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2315           
  Branches       358       358           
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  Hits          2315      2315           

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