Add replicator support

CHANGES/648.feature

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+Added Pulp replication support for Python distributions.

pulp_python/app/replica.py

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+from pulpcore.plugin.replica import Replicator
+
+from pulp_glue.python.context import (
+    PulpPythonDistributionContext,
+    PulpPythonPublicationContext,
+    PulpPythonRepositoryContext,
+)
+from pulp_python.app.models import PythonDistribution, PythonRemote, PythonRepository
+from pulp_python.app.tasks import sync as python_sync
+
+
+class PythonReplicator(Replicator):
+    repository_ctx_cls = PulpPythonRepositoryContext
+    distribution_ctx_cls = PulpPythonDistributionContext
+    publication_ctx_cls = PulpPythonPublicationContext
+    app_label = "python"
+    remote_model_cls = PythonRemote
+    repository_model_cls = PythonRepository
+    distribution_model_cls = PythonDistribution
+    distribution_serializer_name = "PythonDistributionSerializer"
+    repository_serializer_name = "PythonRepositorySerializer"
+    remote_model_serializer_name = "PythonRemoteSerializer"
+    sync_task = python_sync
+
+    def remote_extra_fields(self, upstream_distribution):
+        # This could be dangerous since python remotes are default 'on_demand'
+        # Upstream sources could be entirely 'on_demand' and this replicator policy would heavily
+        # strain the upstream Pulp.
+        return {"policy": "immediate", "prereleases": True}
+
+    def repository_extra_fields(self, remote):
+        # Use autopublish since publications result in faster serving times
+        return {"autopublish": True}
+
+    def sync_params(self, repository, remote):
+        return {"remote_pk": str(remote.pk), "repository_pk": str(repository.pk), "mirror": True}
+
+
+REPLICATION_ORDER = [PythonReplicator]