Revert requiring a specific version of Twisted for mypy checks. (#9618)

changelog.d/9618.misc

@@ -0,0 +1 @@
+Fix incorrect type hints.

tox.ini

@@ -189,7 +189,5 @@ commands=
 [testenv:mypy]
 deps =
     {[base]deps}
-    # Type hints are broken with Twisted > 20.3.0, see https://github.com/matrix-org/synapse/issues/9513
-    twisted==20.3.0
 extras = all,mypy
 commands = mypy