[pyupgrade] synapse/ (#10348)

This PR is tantamount to running 
```
pyupgrade --py36-plus --keep-percent-format `find synapse/ -type f -name "*.py"`
```

Part of #9744

synapse/util/caches/lrucache.py

@@ -90,8 +90,7 @@ def enumerate_leaves(node, depth):
         yield node
     else:
         for n in node.values():
-            for m in enumerate_leaves(n, depth - 1):
-                yield m
+            yield from enumerate_leaves(n, depth - 1)
 
 
 P = TypeVar("P")

synapse/util/caches/treecache.py

@@ -138,7 +138,6 @@ def iterate_tree_cache_entry(d):
     """
     if isinstance(d, TreeCacheNode):
         for value_d in d.values():
-            for value in iterate_tree_cache_entry(value_d):
-                yield value
+            yield from iterate_tree_cache_entry(value_d)
     else:
         yield d