Creation from Excel Serial Dates

[anishnya]

Pull Request Checklist

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• [x] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [x] 📚 Updated documentation for changed code.
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Description of Changes

Closes: #977. New support for creation of Arrow objects from Excel Serial Dates. There seems to be an issue with the linting of the docs, after some tests have been run. Not too sure what it is at first glance.

[codecov[bot]]

Codecov Report

Merging #979 (9e0cb4f) into master (119bb67) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #979   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2054      2064   +10     
  Branches       330       332    +2     
=========================================
+ Hits          2054      2064   +10     

Impacted Files	Coverage Δ
arrow/arrow.py	100.00% <100.00%> (ø)

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[anishnya]

Would this PR be incorporating the decimal portion of the excel serial?

Yes it would. I can add test cases to make it more clear.

[jadchaar]

Rather than creating a new function, I think it may be more consistent with our existing get() interface to add this as a keyword argument like arrow.get(..., excel: bool, default_windows_date: bool). If the excel kwarg is present, we parse the timestamp as an excel timestamp (and account for the default_windows_date kwarg as well), else just parse the input time string as normal.

[systemcatch]

@jadchaar do you think that arrow.get() might have too many kwargs if we add this?