Added support python 3.9

[ermakov-oleg]

Resolves issue #22

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it! and we'll verify it.

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What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
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[ermakov-oleg]

@googlebot I signed it!

[mctavish]

Thanks for the pull request! Unfortunately I'm not able to merge it directly; our build & release pipeline relies on a source of truth other than this github repo. What I'll do is take this set of changes, apply them to the "true" codebase (along with updating our tests) and then get this into a release asap.

Thanks again, and sorry that this isn't as direct as it could be.

[jturmel]

Thank you for the update, this is still holding us up from upgrading our App Engine runtimes to Python 3.9 since we rely on debugger support.

[mctavish]

This PR has been modified and committed as 67e3b8c

Thanks again for the contribution; I'll close this PR now.