Fix multiprocessing Connection import error on OSX

[jhtimmins]

Currently, dag_processing.py imports the multiprocessing library, and uses multiprocessing.connection.Connection to declare a Connection type for the DagFileProcessorManager class. This causes an import error on OS X.

This happens because the multiprocessing package does not expose the .connectionmodule by default. To access this behavior, you need to call multiprocessing.allow_connection_pickling() first.

Alternatively, instead of importing multiprocessing calling multiprocessing.connection.Connection in your code, you can import the Connection class directly:

from multiprocessing.connection import Connection

This PR makes that update.Presumably, this distinction matters bc of weird rules in Python's import code.

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[ashb]

This only affects Py3.8+ right?

[jhtimmins]

@ashb No, it comes up in (at least) 3.7.7 as well.

[mik-laj]

This problem also occurs on Linux.
https://github.com/apache/airflow/runs/644247542

[kaxil]

Makes me wonder, what changed !!

[kaxil]

Most likely was introduced in Python 3.7.7

[kaxil]

Similar issues:

• awswrangler import error with multiprocessing docker-library/python#461
• awswrangler import error on python:3.7 dockers aws/aws-sdk-pandas#151