Cleanup tech debt from removing Python 3.7 and 3.8

[chalmerlowe]

While removing Python 3.7 and 3.8 as dependencies, a number of minor blockers or bits of tech debt were discovered. Resolve or disposition the following items:

• Add a test regarding range_timestamp_dtype. See TODO @ ~#L340. python-bigquery/google/cloud/bigquery/_pandas_helpers.py
• Add a test regarding dataframe_column_dtypes. See TODO @ ~#L648. python-bigquery/google/cloud/bigquery/test_query_pandas.py
• Add a test regarding dataframe_nullable_scalars. See TODO @ ~#L37. python-bigquery/google/cloud/bigquery/test_table_pandas.py
• Identify the correct type hinting for the code associated with the TODO statements:
  • python-bigquery/google/cloud/bigquery/external_config.py @ ~#L838.
  • python-bigquery/google/cloud/bigquery/job/base.py @ ~#L418
  • python-bigquery/google/cloud/bigquery/routine.py @ ~#L521, ~#L528, and ~#L535
  • python-bigquery/google/cloud/bigquery/schema.py @ ~#L236
  • python-bigquery/google/cloud/bigquery/table.py @ ~#L138
  • python-bigquery/google/cloud/bigquery/schema.py @ ~#L236

[chalmerlowe]

PR #2206 should fix all the items under:

Identify the correct type hinting for the code associated with the TODO statements...

[chalmerlowe]

Only partially complete. Reopening.