python: optional unit_noextras session, possibly system_noextras too

[tswast]

Is your feature request related to a problem? Please describe.

There have been times when a BigQuery "extra" (optional dependency) has accidentally become a required dependency. Tests did not catch this because the tests always installed all the optional dependencies it could for a given Python version.

Describe the solution you'd like

A unit_noextras session similar to the one provided here: https://github.com/googleapis/python-bigquery/blob/dcb8728c12f5ab0d7809a1b6cf72755dff973772/noxfile.py#L106

It can optionally be included in the default for projects where there is a risk of making an extra non-optional, such as with tqdm in https://github.com/googleapis/python-bigquery-pandas

Describe alternatives you've considered

One could use one of the middle Python versions to test with no extras via unit_test_extras_by_python. The minimum version should always have all extras, since that's where the constraints for testing minimum versions are expected to be.

This feels a bit arbitrary to me, though is a good workaround for now.

Additional context

• chore: add unit test nox session without extras installed python-bigquery#623 -- add unit_noextras
• DB-API depends on pyarrow when decimal query parameters are used python-bigquery#549 -- accidentally making pyarrow required

[tswast]

Actually, in my testing with googleapis/python-bigquery-pandas#456, the "alternative" using unit_test_extras_by_python doesn't work. It tries to install with pip install -e .[], and pip says .[] is not a valid editable requirement.

Edit: including the following for now

# Workaround for https://github.com/googleapis/synthtool/issues/1317
s.replace(
    ["noxfile.py"], r'extras = "\[\]"', 'extras = ""',
)