Adding a deprecation warning for Python 2.7 and <=3.5.

[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

Adding a deprecation warning for Python 2.7 and <=3.5.

[codecov]

Codecov Report

Merging #828 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #828   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines         1770      1771    +1     
  Branches       304       304           
=========================================
+ Hits          1770      1771    +1     

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

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[systemcatch]

Hey guys do we want the warning in the constructor? That could potentially result in warning messages everytime the class in used.

Why don't we have it on L21-22, that way it'll only trigger on import.

[jadchaar]

Looks good!