Allow codes like ABC123 for flake8 v3 plugins

[peterjc]

Quoting the flake8 plugin author instructions:

"Please Note: Your entry point does not need to be exactly 4 characters as of Flake8 3.0. Consider using an entry point with 3 letters followed by 3 numbers (i.e. ABC123 )."

http://flake8.pycqa.org/en/latest/plugin-development/registering-plugins.html

This change was in last part motivated by the increasing number of error code collisions with flake8 plugins using the original one letter and three numbers pattern.

This regular expression fix was suggested by @jraygauthier on issue #4074.

For #

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)

[msftclas]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

❌ peterjc sign now
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[peterjc]

I don't think I can sign the CLA given any copyright etc from this change belongs to @jraygauthier

[codecov]

Codecov Report

Merging #4443 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4443   +/-   ##
======================================
  Coverage      57%     57%           
======================================
  Files         364     364           
  Lines       15501   15501           
  Branches     2410    2410           
======================================
  Hits         8757    8757           
  Misses       6163    6163           
  Partials      581     581

[jraygauthier]

@peterjc thanks for the PR. This is a minor change, I abandon any copyright (if any) on this change.