Problems with BigQueryIO in python using 2.3.0

[Vilhelm von Ehrenheim]

Hi all!
Not sure if I'm missing something or if this is a bug. I get the following
error in BigQueryIO when using
`beam.io.gcp.bigquery.parse_table_schema_from_json()`:

```
  File
"/home/while/git_repos/motherbrain-cyrano/python_sdk/venv/lib/python2.7/site-packages/apache_beam/io/gcp/bigquery.py",
line 249, in parse_table_schema_from_json


    fields = [_parse_schema_field(f) for f in json_schema['fields']]

  File
"/home/while/git_repos/motherbrain-cyrano/python_sdk/venv/lib/python2.7/site-packages/apache_beam/io/gcp/bigquery.py",
line 236, in _parse_schema_field


    schema = bigquery.TableFieldSchema()

AttributeError: 'module' object has no attribute 'TableFieldSchema'
```

This was working in previous versions.

Regards,
Vilhelm von Ehrenheim