Python 2.7 & 3.5 Deprecation

[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

WIP of Python 2.7 and 3.5 deprecation as outlined in #739.

• Update setup.py classifiers and python_requires to declare arrow only supports Python 3.6+
• Drop Python < 3.6 from CI
• Run pyupgrade --py3-plus
• Convert all formatting to f-strings
• Implement type checking
• Remove all future imports
• Remove any remaining python 2 import shadowing
• Audit and overhaul requirements file
• Change timestamp formatting to use datetime.timestamp()
• Replace hardcoded MAX_TIMESTAMP constant with datetime.max.timestamp()
• Convert isinstance(value, numbers.Integral) to isinstance(value, int)
• Remove isstr function