Use slots in attrs classes where possible (#8296)

slots use less memory (and attribute access is faster) while slightly
limiting the flexibility of the class attributes. This focuses on objects
which are instantiated "often" and for short periods of time.

synapse/http/federation/well_known_resolver.py

@@ -311,7 +311,7 @@ def _parse_cache_control(headers: Headers) -> Dict[bytes, Optional[bytes]]:
     return cache_controls
 
 
-@attr.s()
+@attr.s(slots=True)
 class _FetchWellKnownFailure(Exception):
     # True if we didn't get a non-5xx HTTP response, i.e. this may or may not be
     # a temporary failure.

synapse/http/matrixfederationclient.py

@@ -76,7 +76,7 @@ MAXINT = sys.maxsize
 _next_id = 1
 
 
-@attr.s(frozen=True)
+@attr.s(slots=True, frozen=True)
 class MatrixFederationRequest:
     method = attr.ib()
     """HTTP method