Support use local file path as value of PEP_ARTIFACT_URL

[mozillazg]

I think we can set settings.PEP_ARTIFACT_URL to os.path.join(settings.BASE, 'peps/tests/peps.tar.gz') in settings/local.py. This way contributors wouldn't need to clone python/peps repo if they would like to work on our PEP converter code.
---- @berkerpeksag #1412 (comment)

@berkerpeksag What do you think about this PR? Is it necessary?

[berkerpeksag]

I think we need to change the following line:

pythondotorg/pydotorg/settings/local.py

Lines 27 to 28 in f1747e7

	# Set the URL to where to fetch PEP artifacts from
	PEP_ARTIFACT_URL = 'https://pythondotorg-assets-staging.s3.amazonaws.com/fake-peps.tar.gz'

to

PEP_ARTIFACT_URL = os.path.join(BASE, 'peps/tests/peps.tar.gz')

[berkerpeksag]

Thank you!