mautrix-telegram: Add override for mautrix

Due to tight version constraints on the mautrix library, this commit
introduces an override pinning the exact version necessary.

pkgs/servers/mautrix-telegram/default.nix

@@ -17,6 +17,14 @@ let
         };
         doCheck = false;
       });
+      mautrix = super.mautrix.overridePythonAttrs (oldAttrs: rec {
+        version = "0.20.0";
+        src = oldAttrs.src.override {
+          rev = "refs/tags/v${version}";
+          hash = "sha256-op28CGpJBcCBiy0WXboaf4JeNRIMX6653QkAV6XW/yI=";
+        };
+        doCheck = false;
+      });
     };
   };
 in