BigQuery IO does not support bytes in Python 3

In Python 2 you could write bytes data to BigQuery. This is tested in
https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/big_query_query_to_table_it_test.py#L186

Python 3 does not support

json.dumps({'test': b'test'})

which is used to encode the data in
https://github.com/apache/beam/blob/master/sdks/python/apache_beam/io/gcp/bigquery_tools.py#L959

How should writing bytes to BigQuery be handled in Python 3?

• Forbid writing bytes into BigQuery on Python 3
• Guess the encoding (utf-8?)
• Pass the encoding to BigQuery

cc: tvalentyn