THRIFT-5537: Remove python2

[jimexist]

Remove python2

• Closes #2585

• [x] Did you create an Apache Jira ticket? (not required for trivial changes)
• [x] If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• [x] Did you squash your changes to a single commit? (not required, but preferred)
• [x] Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• [x] If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[fishy]

I think you still need to sort out some CI issues, but please ping me when you are ready and I'll be happy to review it. thank you for doing it!

[jimexist]

Thanks for doing this!

Those seem to be new errors related to python:

• https://app.travis-ci.com/github/apache/thrift/jobs/582437473
• https://app.travis-ci.com/github/apache/thrift/jobs/582437474
• https://app.travis-ci.com/github/apache/thrift/jobs/582437475
• https://app.travis-ci.com/github/apache/thrift/jobs/582437476

@fishy turns out cmake was picking up python2 instead of python3 for these test runs. however i cannot get cmake version > 3.10 because by default it's the one shipped in bionic. i'm trying to upgrade to focal in #2528 but before that i guess this PR is a bit stuck unless we want to drop python tests in cmake for the moment