Set SERVE_FROM_PUBLICATION on PythonDistribution

I don't know if this is strictly required since pulp_python defines its
own content handler function but SERVE_FROM_PUBLICATION ought to be set
from a technical correctness standpoint. Also future work might expect
it to be set such as the repo version protection PR I have open against
pulpcore:

pulp/pulpcore#4750

[noissue]

pulp_python/app/models.py

@@ -52,6 +52,7 @@ class PythonDistribution(Distribution):
     """
 
     TYPE = "python"
+    SERVE_FROM_PUBLICATION = True
 
     allow_uploads = models.BooleanField(default=True)