Several improvements for consistency of Python2 / Python3 conversions.

[tvalentyn]

[BEAM-1251]
Use (str, future.builtins.unicode) where six.string_types was used in the past.
For Py2/Py3 compatibility, add complimentary ne methods which are autogenerated in Py3.
Use past.builtins.unicode where six.text_type was used.
Use bytes where six.binary_type was used.

Post-Commit Tests Status (on master branch)

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

[tvalentyn]

R: @Fematich , @udim

[udim]

Thanks!

[tvalentyn]

@Fematich would you have time to take a look at this PR before next week?

[Fematich]

Looks good to me! I was only wondering if the change from six.binary_type to bytes on line 177 in pubsub.py could introduce issues for the coders, but this should be equivalent.

[udim]

@Fematich six.binary_type is str and bytes is str in Python 2.7, so it's equivalent for existing code.

[tvalentyn]

Thanks, @Fematich and @udim.