AVRO-2676: Document that lang/py3 will be deprecated

[sekikn]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Avro Jira issues and references them in the PR title. For example, "AVRO-1234: My Avro PR"
  • https://issues.apache.org/jira/browse/AVRO-2676
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

No additional test, since it's a documentation fix. Instead, I ran ant in the doc directory and confirmed the documents were generated as expected.

Commits

• My commits all reference Jira issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • All the public functions and the classes in the PR contain Javadoc that explain what it does

[kojiromike]

It isn't quite supposed to be api compatible. For whatever reason, avro-python3 is a little different, particularly with capitalizing some names like Parse. But it's mostly API compatible.

[sekikn]

@kojiromike Thanks! Updated the description.

[Fokko]

Restarted the CI, seems like an unrelated issue