Default to UTC for arrow.fromtimestamp()

[krisfremen]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Default to UTC for arrow.fromtimestamp()

[codecov]

Codecov Report

Merging #1036 (468b2ed) into master (246c55d) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1036   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2163      2163           
  Branches       342       342           
=========================================
  Hits          2163      2163           

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

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[anishnya]

LGTM!