Fix synapse.config.__main__ on python 3 (#4356)

changelog.d/4356.bugfix

@@ -0,0 +1 @@
+Fix `python -m synapse.config` on Python 3.

synapse/config/__main__.py

@@ -16,7 +16,7 @@ from synapse.config._base import ConfigError
 
 if __name__ == "__main__":
     import sys
-    from homeserver import HomeServerConfig
+    from synapse.config.homeserver import HomeServerConfig
 
     action = sys.argv[1]