Canonical redirects: check if the project supports custom domains (#10098)

In .com need to check for this.
But we don't allow users to create custom domains if they don't have
the right subscription, so not sure if this is really needed..
It can be useful only if a user downgrades their subscription I guess.

Author: stsewd

readthedocs/proxito/views/serve.py

@@ -14,7 +14,7 @@
 from readthedocs.builds.constants import EXTERNAL, INTERNAL, LATEST, STABLE
 from readthedocs.builds.models import Version
 from readthedocs.core.mixins import CDNCacheControlMixin
-from readthedocs.core.resolver import resolve_path
+from readthedocs.core.resolver import resolve_path, resolver
 from readthedocs.core.unresolver import (
     InvalidExternalVersionError,
     InvalidPathForVersionedProjectError,
@@ -281,7 +281,8 @@ def _get_canonical_redirect_type(self, request):
                 .filter(canonical=True, https=True)
                 .exists()
             )
-            if canonical_domain:
+            # For .com we need to check if the project supports custom domains.
+            if canonical_domain and resolver._use_cname(project):
                 log.debug(
                     "Proxito Public Domain -> Canonical Domain Redirect.",
                     project_slug=project.slug,