Allow code like ABC123 for flake8 v3 plugin

[dbftdiyoeywga]

For #4074

This PR change the regular expression for linter.
It is able to show flake8-plugin's output in PROBLEMS panel.

• 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!)
• Appropriate comments and documentation strings in the code
• 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)
• The wiki is updated with any design decisions/details.

Special thanks for @jraygauthier and @peterjc(#4443).