In DomainSpecificString, override __repr__ in addition to __str__

For some reason, string interpolation on a DomainSpecificString object
like "%r" % (domainSpecificStringObj) fails under PyPy, because the
default __repr__ implementation wants to iterate over the object. I'm
not sure why that happens, but overriding __repr__ instead of __str__
fixes this problem, and is arguably the more appropriate thing to do
anyways.

synapse/types.py

@@ -169,7 +169,7 @@ def is_valid(cls, s):
         except Exception:
             return False
 
-    __str__ = to_string
+    __repr__ = to_string
 
 
 class UserID(DomainSpecificString):