AVRO-3004 Drop support for Python 3.5

[subhashb]

This PR removes 3.5 support from build and setup, including classifiers

Closes: AVRO-3004 (https://issues.apache.org/jira/browse/AVRO-3004)

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• My PR addresses the issue "AVRO-3004 Drop support for Python 3.5"
  • https://issues.apache.org/jira/browse/AVRO-3004

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  NA

[kojiromike]

Can we add 3.9 now? I think it will just work.

Also, we might need to remove 3.5 and add 3.9 to the GitHub actions matrix

[subhashb]

Can we add 3.9 now? I think it will just work.

Also, we might need to remove 3.5 and add 3.9 to the GitHub actions matrix

Done and tests pass.
Actually, Github actions were not testing for 3.5 and in turn had pypy3.7 tests. I have added pypy3.7 to tox envs.

[RyanSkraba]

This almost LGTM -- it seems the docker image that we use for releases (based on debian stretch) is still 3.5.3 and there isn't any 3.6 available for it.

You can merge this safely now, but the next release is going to need a fix to the environment!

[kojiromike]

@RyanSkraba the mega base image is always going to be a sticking point. We have looked at nektos/act as a substitute, but it isn't free from trouble.

[RyanSkraba]

For reference, I created https://issues.apache.org/jira/browse/AVRO-3092 to discuss that issue and PR#1155 for a quick fix! But we can probably do better for a long-term solution.