BigQuery: Set IPython user agent when running queries with IPython cell magic

[plamut]

Closes #8696.

How to test

See issue description - the Client should be instantiated with a custom user agent string when used from an IPython cell.

Things to discuss

• A connection from the context could override this custom user agent string - is that expected behavior, or should the cell magic actually override the user agent string in context connections, too?
  No, this is primarily used with Kaggle datasets, thus the overridden user agent string should take precedence.

[tswast]

LGTM.

Test failures might be from an update to a dependency (pyarrow) that causes more warnings to be raised than expected.

We should see what those new warnings are and update the test to not care quite so much about them if they're safe to ignore.

A connection from the context could override this custom user agent string - is that expected behavior, or should the cell magic actually override the user agent string in context connections, too?

We added that _connection override feature for Kaggle public datasets, since they proxy connections to BigQuery. We have other ways to determine which requests come from Kaggle, and if anyone is proxying like Kaggle is doing, arguably we should keep whatever user-agent they have set, as it will be more specific than ours.

[plamut]

The three new deprecation warnings that caused one of the test to fail were related to using some deprecated Pandas thingies, namely the RangeIndex attributes _start, _stop, and _step - these do not concern us?

I modified the test to only focus on the expected warning related to pyarrow.

[plamut]

Heh, now the coverage check is complaining. Will check and make the necessary adjustments.

[plamut]

The coverage check complained, because not all paths have been taken in the next(...) construct in the test, go figure.

However, I did notice that even those three new warnings are caused by pyarrow. That was my bad, I initially only looked at w.message, but str(w) contains more info, e.g. the filename of the warning source, and that source is the pyarrow lib itself:

"{message : DeprecationWarning('RangeIndex._start is deprecated and will be removed in a future version. Use RangeIndex.start instead',), category :                     
'DeprecationWarning', filename : '/home/.../bigquery/.nox/unit-3-6/lib/python3.6/site-packages/pyarrow/pandas_compat.py', lineno : 383, line : None}"