refactor: omit read_session with latest google-cloud-bigquery-storage

[tswast]

read_session is unnecessary as of google-cloud-bigquery-storage>=2.6.0.
This will allow us to more loudly deprecate the use of rows(read_session). googleapis/python-bigquery-storage#229

Rather than require 2.6.0, version switches will allow us to keep our
requirements range wider. Will want to give this version some time to bake
before making it the minimum version. #747

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Fixes #742 🦕

[plamut]

This could potentially be tricky, because the value of the _IS_BQ_STORAGE_READ_SESSION_OPTIONAL "constant" depends on whether _verify_bq_storage_version() has been called or not.

It would IMO be more straightforward if we performed version detection at import time and already make the final decision on the value of _IS_BQ_STORAGE_READ_SESSION_OPTIONAL then.

[tswast]

We don't import google.cloud.bigquery_storage in _download_table_bqstorage_stream. Since that function gets a client and other objects passed in to it.

How about we make sure we call _verify_bq_storage_version() from _download_table_bqstorage_stream() and optimize to not call the version parsing too often?

[plamut]

The overhead of parsing a version is most likely negligible compared to the request/response times, I was more concerned that changing a module "constant" might represent a landmine in the "right" constellation of the planets.

Perhaps just adding a comment that this private "constant" is not a constant and can change when calling _verify_bq_storage_version() would be enough? You know, just to warn the readers about it.

[tswast]

Good point. I think there's a way to do this without "constants" that aren't actually constant. I think maybe I'll create a little class to hold our version comparisons.

[tswast]

I wanted to add type annotations, but it turns out pkg_resources is just a thin wrapper around packaging.

https://github.com/pypa/setuptools/blob/a4dbe3457d89cf67ee3aa571fdb149e6eb544e88/pkg_resources/__init__.py/#L112

[plamut]

Looks good and clean!

Consider maybe adding a few unit tests directly testing the BQStorageVersions class, although it does seem that the class is tested indirectly through other tests (the coverage does not complain).

Edit: I mean, the coverage check does complain, but not about _helpers.BQStorageVersions.

[plamut]

This version checker, sir, is classy (pun intended). Well done!

[plamut]

Oops, I now noticed you copied my typo, let's remove it.

Suggested change

	in setup.py, the the calling code can use this helper to verify the version
	in `setup.py`, the calling code can use this helper to verify the version

found a typo

[tswast]

Turns out the coverage error was due to my use of a lambda that never executed. I'll remove those and add a few tests for the underlying class in my next commit.