fix version number

we're not doing a final release yet!

CHANGES.md

@@ -1,5 +1,5 @@
-Synapse 1.21.0 (2020-10-01)
-===========================
+Synapse 1.21.0rc1 (2020-10-01)
+==============================
 
 Features
 --------

synapse/__init__.py

@@ -48,7 +48,7 @@ try:
 except ImportError:
     pass
 
-__version__ = "1.21.0"
+__version__ = "1.21.0rc1"
 
 if bool(os.environ.get("SYNAPSE_TEST_PATCH_LOG_CONTEXTS", False)):
     # We import here so that we don't have to install a bunch of deps when