Fix arrow.get() crashing when tzinfo=None is explicitly passed

[aoright]

Fixes #1259

When arrow.get() is invoked with tzinfo=None, the tzinfo kwarg is placed in kwargs with a value of None. In factory.py, the condition if len(kwargs) == 1 and tz is None: assumes that if tz is None it means the user omitted tzinfo. However, if the user explicitly provided tzinfo=None, this triggers the condition to true and redirects the call to the Arrow constructor which throws TypeError.

This PR fixes the check to if len(kwargs) == 1 and "tzinfo" not in kwargs:, properly distinguishing between an omitted tzinfo kwarg and an explicit tzinfo=None kwarg.

Tests have been added.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (e6a8878).

Additional details and impacted files

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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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