Use a specific version of pycodestyle to resolve broken CI tests

[DonJayamanne]

Fixes #1350

This pull request:

• Has a title summarizes what is changing
• Includes a news entry file (remember to thank yourself!)
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1366 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1366      +/-   ##
==========================================
- Coverage   71.23%   71.11%   -0.13%     
==========================================
  Files         269      269              
  Lines       12403    12403              
  Branches     2199     2199              
==========================================
- Hits         8835     8820      -15     
- Misses       3437     3448      +11     
- Partials      131      135       +4

Impacted Files	Coverage Δ
...reter/locators/services/cacheableLocatorService.ts	91.83% <0%> (-4.09%)	⬇️
src/client/debugger/PythonProcess.ts	47.91% <0%> (-3.34%)	⬇️
...nterpreter/locators/services/currentPathService.ts	95.23% <0%> (-2.39%)	⬇️
src/client/common/process/pythonProcess.ts	84% <0%> (-2%)	⬇️
src/client/interpreter/display/index.ts	98.11% <0%> (-1.89%)	⬇️
src/client/interpreter/interpreterService.ts	76.34% <0%> (-1.08%)	⬇️
src/client/debugger/mainV2.ts	76.44% <0%> (-0.83%)	⬇️
src/client/linters/baseLinter.ts	87.5% <0%> (ø)	⬆️
src/client/linters/flake8.ts	100% <0%> (+8.33%)	⬆️

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