deps: Work around release build failure in macOS

Author: palango

requirements/requirements-ci.txt

@@ -304,7 +304,7 @@ idna==2.8
     #   requests
     #   twisted
     #   yarl
-importlib-metadata==4.8.1
+importlib-metadata==4.2.0
     # via
     #   -r requirements-dev.txt
     #   pluggy

requirements/requirements-dev.in

@@ -59,3 +59,6 @@ bump2version
 # Test support
 matrix-synapse
 six>=1.13.0  # work around bad deps declaration in treq, see https://github.com/twisted/treq/commit/934e127a2b915bf02be86d23f2cf8a65bcdb2533#r42782781
+
+# Mac build failure
+importlib-metadata<4.3