Travis CI: Ubuntu Trusty has reached end of standard support

[cclauss]

https://wiki.ubuntu.com/Releases

Also, upgrade lint_python_base from Python 3.6 to 3.9 to find more issues.

What kind of change does this PR introduce?

• bug fix
• feature
• docs update
• tests/coverage improvement
• refactoring
• other

What is the related issue number (starting with #)

What is the current behavior? (You can also link to an open issue here)

What is the new behavior (if this is a feature change)?

Other information:

Checklist:

• I think the code is well written
• I wrote good commit messages
• I have squashed related commits together after the changes have been approved
• Unit tests for the changes exist
• Integration tests for the changes exist (if applicable)
• I used the same coding conventions as the rest of the project
• The new code doesn't generate linter offenses
• Documentation reflects the changes
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences