Fix x formatting token on Python 2.7

[jadchaar]

Pull Request Checklist

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

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

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Description of Changes

Fix bug with Python 2.7 and the new "x" token.

[codecov-io]

Codecov Report

Merging #781 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #781   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1695      1695           
  Branches       289       289           
=========================================
  Hits          1695      1695           

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

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