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 -

django-silk/silk/code_generation/django_test_client.py

Lines 45 to 48 in ed3e326

	return autopep8.fix_code(
	t.render(Context(context)),
	options=autopep8.parse_args(['--aggressive', '']),
	)

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