Accept Decimal timestamps in fromtimestamp and utcfromtimestamp

[Nishuuzz]

arrow.get() accepts a Decimal timestamp, but the constructors it stands in for reject the same value:

>>> arrow.get(Decimal("1591328104.308505"))
<Arrow [2020-06-05T03:35:04.308505+00:00]>

>>> Arrow.utcfromtimestamp(Decimal("1591328104.308505"))
ValueError: The provided timestamp Decimal('1591328104.308505') is invalid.

Same for Arrow.fromtimestamp.

The factory converts a Decimal to a float before dispatching, and test_one_arg_decimal covers it, so this is supported input rather than something that happens to work. fromtimestamp and utcfromtimestamp instead gate on util.is_timestamp, which only admits int, float and str. A Decimal is turned away as "invalid" — and the very next line calls float(timestamp), which would have been perfectly happy with it.

This converts it in those two methods, the same way the factory already does, and widens their type hints and docstrings to match.

I deliberately did not change util.is_timestamp, even though that is where the check lives. #1321 and #1324 are both open against that function, and it is public API in its own right, so it seemed better to leave it alone and fix the asymmetry where it actually shows.

Testing

test_fromtimestamp_decimal and test_utcfromtimestamp_decimal, next to the existing tests for those constructors. Both fail on master with the ValueError above.

Full suite is green at 1903 passed, and black and flake8 are clean. The existing float, str and invalid-input behaviour is unchanged — "invalid timestamp" still raises, and the two existing assertions covering that still pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (be3fded).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1340   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2320    +5     
  Branches       358       360    +2     
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+ Hits          2315      2320    +5     

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