[BEAM-1251] Replace NameError-driven dispatch with past

[superbobry]

The only leftover is the ToStringCoder which uses NameError to detect Python2/3. The logic there looks fishy (it encodes text data on 2.X but not on 3.X), so I'd rather address it in a follow up PR.

Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---		---	---	---	---

[cclauss]

This is great stuff... Can you just try:

• flake8 . --count --show-source --statistics --select=E901,E999,F821,F822,F823 and try to whack out as many of those lingering issues as you can?

We still have
4 E999 SyntaxError: invalid syntax
11 F821 undefined name 'buffer'

Leave that 'buffer' one to me... I have nice progress on that one but all other are fair game.

[superbobry]

Will do. I can fix buffer if you are short on time, just let me know.

@cclauss also, can we merged futurization PRs first?

[aaltay]

Thank you @superbobry.

Could you coordinate your changes with rest of the people working on python 3 changes on the mailing list. There was a recent reviewed proposal for python 3 conversion and some of the bits you are removing were added as part of it. You can find the proposal on the Beam web site: https://beam.apache.org/contribute/#python-3-support

@tvalentyn could also help with coordination.

(I noticed that you are already started coordinating on the mailing list. That is great, please share major planned changes, such as the use of past in the mailing list.)

[cclauss]

Please resolve conflict.

[superbobry]

@aaltay thanks for the feedback. I've added a relevant comment on the migration proposal doc.

@cclauss done.

[tvalentyn]

Thanks, @superbobry , this LGTM. I also ran this past coders microbenchmark in #5565 and did not see any performance impact.
@RobbeSneyders, do you have any concerns?

[RobbeSneyders]

LGTM, thank you.
We discussed this change in the io PR here. I originally proposed to use the NameError-driven approach because of problems with typechecks when importing from builtins. However, it seems that imports from past.builtins import the native types and don't cause these problems.

[superbobry]

Rebased on top of master.

[aaltay]

Thank you all for making this happen!