chore: Remove python patch version in AppVeyor to be more robust

[aahung]

Which issue(s) does this change fix?

Why is this change necessary?

When AppVeyor updates their python env, like from 3.7.7 to 3.7.9, 3.7.7 won't be available, and builds will fail due to the error:

/opt/appveyor/build-agent/bash-shell.sh: line 51: /home/appveyor/venv3.7.7/bin/activate: No such file or directory

According to AppVeyor doc, we don't have to specify patch version: https://www.appveyor.com/docs/linux-images-software/#python

How does it address the issue?

Remove patch version so it can handle patch version python upgrade on AppVeyor side.

What side effects does this change have?

nothing

Checklist

• Write design document (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• make update-reproducible-reqs if dependencies were changed
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.