chore(bigquery): run unit tests with Python 3.8

[tswast]

I had to decrease the required per-version coverage on account of the missing pyarrow dependency.

Towards #9878

[tswast]

I've filed https://github.com/googleapis/google-cloud-python/issues/9881 to track removing the Python 3.8-specific updates in the noxfile once all dependencies are available.

[plamut]

Seems good, and the tests run under Python 3.8, albeit with a lot of them skipped for the time being.

We might need to lower the coverage threshold, though, I got a coverage failure when running the session unit-3.8 (total coverage 92.90%, required 93%). That was after removing the .coverage file first.

[plamut]

The additional comments look good. 👍

However, I now cannot verify the PR, as the nox setup phase fails with numpy import errors, and complaining that LLVM version 7.0+ is needed (I seem to have 6.0.0 installed).

Could be just my specific environment, but I did not dig further. Could perhaps somebody else try on their machine?

BTW, the coverage check (still) seems to fail on Kokoro...

[tswast]

From Kokoro:

Session unit-3.8 skipped: Python interpreter 3.8 not found.

I'll send a PR to the test containers repo to see if I can get this in our multi-Python image. googleapis/testing-infra-docker#44