[docs] Contradictory information about what versions of python are supported

[Jennifer88huang-zz]

Describe the bug
There is contradictory information about what versions of python are supported.

1. It says that versions 3.4-3.7 are supported at http://pulsar.apache.org/docs/en/client-libraries-python/
2. It says that only version 2.7 is supported at http://pulsar.apache.org/api/python/
   The info is out-of-date and needs regenerating.
3. python3 now being supported, there is other new functionality
   which is not shown: e.g.

• Client.create_producer and Client.create_reader can take a "schema"
  argument
• Message.value() contains the object deserialized via schema

Note: The issue is found and reported by Brian (b.candler@pobox.com). Thank Brian for his contribution.

[Jennifer88huang-zz]

@merlimat @jerrypeng @sijie The generated API files are not copied to the right path, could you please help to resolve the issue?

[sijie]

@jennifer88huang I think @tuteng was looking into fixing the API generation. Can you work with him on this?

[Jennifer88huang-zz]

@sijie Sure.

[Anonymitaet]

#1 and #2 are tracked (and fixed) in #5345

[Anonymitaet]

#3 has been fixed, here is the latest doc.

• client.create_producer and client.create_reader can take a "schema"
  argument
  

• Message.value() contains the object deserialized via schema
  

[Anonymitaet]

@jennifer88huang all issues have been fixed, could you please close this issue?

[Jennifer88huang-zz]

@Anonymitaet When you fix an issue, you can follow the template in PR, and write "Fixes #xxx". When the PR is merge, the related issue is closed automatically.