On demand imports failing in pandas.io.gbq
After upgrading to 0.16.0 BigQuery functionality is not working. Upon invoking pd.read_gbq(), the _test_imports() function is running, which in turn runs _importers(). Everything is all well and good up to here, but per pdb, once _importers() returns, the imported modules are not accessible and read_gbq() throws `NameError: global name 'OAuth2WebServerFlow' is not defined`.
