chore(build): apply black formatting + CI step to main codebase

[hydrosquall]

Summary of Changes / Motivation

• Closing out the last two items in [platform] End support for Python 2 + Python 3.5 #601
• Will make it easier for new contributors to get automated feedback about matching the project style guidelines

Checklist

• Update HISTORY.rst with an entry summarizing your change (n/a., not a user facing feature)

[codecov]

Codecov Report

Merging #663 (f153a9c) into master (fe0e199) will not change coverage.
The diff coverage is 89.52%.

@@           Coverage Diff           @@
##           master     #663   +/-   ##
=======================================
  Coverage   90.47%   90.47%           
=======================================
  Files           6        6           
  Lines         294      294           
=======================================
  Hits          266      266           
  Misses         28       28           

Impacted Files	Coverage Δ
tiingo/__version__.py	0.00% <0.00%> (ø)
tiingo/exceptions.py	100.00% <ø> (ø)
tiingo/wsclient.py	68.42% <78.57%> (ø)
tiingo/api.py	93.18% <91.95%> (ø)
tiingo/__init__.py	100.00% <100.00%> (ø)
tiingo/restclient.py	100.00% <100.00%> (ø)

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[hydrosquall]

The codecov decrease in coverage message is a false positive, black changed the length of some files by changing where linebreaks happened, but that doesn't affect the functionality. We can safely ignore it.