matrix-synapse: make dependency for `hiredis` optional

This is only needed for replication which isn't even supported by the
current NixOS module.

pkgs/servers/matrix-synapse/default.nix

@@ -1,5 +1,6 @@
 { lib, stdenv, python3, openssl
 , enableSystemd ? stdenv.isLinux, nixosTests
+, enableRedis ? false
 }:
 
 with python3.pkgs;
@@ -53,14 +54,15 @@ buildPythonApplication rec {
     typing-extensions
     authlib
     pyjwt
-    hiredis
-  ] ++ lib.optional enableSystemd systemd;
+  ] ++ lib.optional enableSystemd systemd
+    ++ lib.optional enableRedis hiredis;
 
   checkInputs = [ mock parameterized openssl ];
 
   doCheck = !stdenv.isDarwin;
 
   checkPhase = ''
+    ${lib.optionalString (!enableRedis) "rm -r tests/replication # these tests need the optional dependency 'hiredis'"}
     PYTHONPATH=".:$PYTHONPATH" ${python3.interpreter} -m twisted.trial tests
   '';