Package 'autopep8' shouldn't need to be required when installing django-silk

[PabloAlexis611]

Since autopep8 is purely for Python code formatting, the installation of autopep8 should be optional/maybe be provided via extras instead. Maybe something like pip install django-silk[dev]

[albertyw]

django-silk supports generating a python snippet to refire a request - https://github.com/jazzband/django-silk/blob/ed3e3261af96dc054fe72b80cd567313f835515b/silk/code_generation/django_test_client.py#L45-L48

This python snippet gets run through autopep8 so autopep8 is a runtime dependency of django-silk, not just a test dependency.