Python 3 pipeline fails with errors in StockUnpickler.find_class() during loading a main session.

When running Apache Beam with Python3 on Google Cloud Dataflow the pipeline fails during pickler.load_session(session_file):
StockUnpickler.find_class(self, module, name) AttributeError: Can't get attribute 'SomeAttribute' on

Note that this is different from BEAM-8651, since the error happens in a Batch Pipeline on a Dataflow runner and the error happens consistently.

When testing it in the local/direct runner there seems to be no issue.
 
```

class FlattenCustomActions(beam.PTransform):
""" Transforms Facebook Day Actions Only retains
actions with custom_conversions
Flattens the actions
Adds custom conversions names
using a side input
"""
def __init__(self, conversions):
super(FlattenCustomActions,
self).__init__()
self.conversions = conversions def expand(self, input_or_inputs):

return (
input_or_inputs
| "FlattenActions" >> beam.ParDo(flatten_filter_actions)

| "AddConversionName" >> beam.Map(add_conversion_name, self.conversions)
)

#
...
# in run():
pipeline_options = PipelineOptions(pipeline_args)
pipeline_options.view_as(SetupOptions).save_main_session
= True
p = beam.Pipeline(options=pipeline_options)
conversions_output = (
p

| "ReadConversions" >> ReadFromText(known_args.input_conversions, coder=JsonCoder())
|
TransformConversionMetadata()
) (
conversions_output
| "WriteConversions"

>> WriteCoerced(
known_args.output_conversions,
known_args.output_type,

schema_path=BIGQUERY_SCHEMA_CONVERSIONS_PATH,
)
) (
p

| ReadFacebookJson(known_args.input, retain_root_fields=True)
| FlattenCustomActions(beam.pvalue.AsList(conversions_output))

| "WriteActions"
>> WriteCoerced(
known_args.output, known_args.output_type,
schema_path=BIGQUERY_SCHEMA_ACTIONS_PATH
)
)
```

I receive the following Traceback in Dataflow:
```

Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/dataflow_worker/batchworker.py",
line 773, in run self._load_main_session(self.local_staging_directory)
File "/usr/local/lib/python3.6/site-packages/dataflow_worker/batchworker.py",
line 489, in _load_main_session pickler.load_session(session_file)
File "/usr/local/lib/python3.6/site-packages/apache_beam/internal/pickler.py",
line 287, in load_session return dill.load_session(file_path)
File "/usr/local/lib/python3.6/site-packages/dill/_dill.py",
line 410, in load_session module = unpickler.load()
File "/usr/local/lib/python3.6/site-packages/dill/_dill.py",
line 474, in find_class return StockUnpickler.find_class(self, module, name) AttributeError: Can't get
attribute 'FlattenCustomActions' on

```

Imported from Jira [BEAM-8441](https://issues.apache.org/jira/browse/BEAM-8441). Original Jira may contain additional context.
Reported by: Jannik.Franz@umusic.com.