Travis CI: Upgrade mainstream_python from 3.7 to 3.9

[cclauss]

Travis passes on:

• Python 3.8.3
• Python 3.9.0

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)
Travis tests are passing against Python 3.7 but are failing on CPython nightly (Python 3.10-dev). This PR attempts to ensure that Travis tests pass on Python 3.8.

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
• 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