BigQuery: coverage failures on master branch.

[tseaver]

From this Kokoro job:

Running session cover
Creating virtualenv using python3.6 in .nox/cover
pip install coverage pytest-cov
coverage report --show-missing --fail-under=100
Name                                        Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------------------------
google/cloud/bigquery/__init__.py              49      0      0      0   100%
google/cloud/bigquery/_helpers.py             219      0    112      0   100%
google/cloud/bigquery/_http.py                 12      0      0      0   100%
google/cloud/bigquery/_pandas_helpers.py       61      0     24      0   100%
google/cloud/bigquery/client.py               476      0    196      0   100%
google/cloud/bigquery/dataset.py              193      0     57      0   100%
google/cloud/bigquery/dbapi/__init__.py        30      0      0      0   100%
google/cloud/bigquery/dbapi/_helpers.py        42      0     26      0   100%
google/cloud/bigquery/dbapi/connection.py      14      0      2      0   100%
google/cloud/bigquery/dbapi/cursor.py         107      0     30      0   100%
google/cloud/bigquery/dbapi/exceptions.py      11      0      0      0   100%
google/cloud/bigquery/dbapi/types.py           22      0      0      0   100%
google/cloud/bigquery/external_config.py      213      0     28      0   100%
google/cloud/bigquery/job.py                 1008      0    207      0   100%
google/cloud/bigquery/magics.py                85      0     16      0   100%
google/cloud/bigquery/model.py                118      0     20      0   100%
google/cloud/bigquery/query.py                215      0     70      0   100%
google/cloud/bigquery/retry.py                 13      0      4      0   100%
google/cloud/bigquery/schema.py                54      0     14      0   100%
google/cloud/bigquery/table.py                654      0    229      0   100%
tests/unit/__init__.py                          0      0      0      0   100%
tests/unit/helpers.py                           8      0      0      0   100%
tests/unit/model/__init__.py                    0      0      0      0   100%
tests/unit/model/test_model.py                112      0      0      0   100%
tests/unit/model/test_model_reference.py       71      0      0      0   100%
tests/unit/test__helpers.py                   597      0      6      0   100%
tests/unit/test__http.py                       58      0      0      0   100%
tests/unit/test__pandas_helpers.py            103      0     38      0   100%
tests/unit/test_client.py                    2345      0     60      0   100%
tests/unit/test_dataset.py                    455      0     18      0   100%
tests/unit/test_dbapi__helpers.py              44      0      6      0   100%
tests/unit/test_dbapi_connection.py            43      0      0      0   100%
tests/unit/test_dbapi_cursor.py               201      0      4      0   100%
tests/unit/test_dbapi_types.py                 18      0      0      0   100%
tests/unit/test_external_config.py            176      0      0      0   100%
tests/unit/test_job.py                       3027      0    126      0   100%
tests/unit/test_magics.py                     327      0      4      1    99%   242->exit
tests/unit/test_query.py                      611      0      4      0   100%
tests/unit/test_retry.py                       38      0      0      0   100%
tests/unit/test_schema.py                     223      0      2      0   100%
tests/unit/test_table.py                     1472      0     70      0   100%
---------------------------------------------------------------------------------------
TOTAL                                       13525      0   1373      1    99%
Command coverage report --show-missing --fail-under=100 failed with exit code 2
Session cover failed.

@tswast I can't run the python2.7 unit tests at the moment locally, because numba (and maybe other deps) don't support the default UCS2 build (see numba/numba#4139), which makes this hard to diagnose.

[tswast]

I'm not able to reproduce locally. I think I may have fixed this in #8179 with commit 7ebfcc5 that used a for loop instead of all.

[tswast]

cover passes in latest commit on master: https://source.cloud.google.com/results/invocations/b57bdacb-177c-4699-8d58-60d6de917f1a/targets