Narrow the importlib_metadata dependency (#12400)

changelog.d/12400.misc

@@ -0,0 +1 @@
+Make missing `importlib_metadata` dependency explicit.

synapse/python_dependencies.py

@@ -91,7 +91,7 @@ REQUIREMENTS = [
     "packaging>=16.1",
     # At the time of writing, we only use functions from the version `importlib.metadata`
     # which shipped in Python 3.8. This corresponds to version 1.4 of the backport.
-    "importlib_metadata>=1.4",
+    "importlib_metadata>=1.4 ; python_version < '3.8'",
 ]
 
 CONDITIONAL_REQUIREMENTS = {