fix: quote WeakKeyDictionary typing for python<=3.7

Author: zevisert

databases/core.py

@@ -35,12 +35,12 @@
 
 logger = logging.getLogger("databases")
 
+
 _ACTIVE_CONNECTIONS: ContextVar[
-    typing.Optional[weakref.WeakKeyDictionary["Database", "Connection"]]
+    typing.Optional["weakref.WeakKeyDictionary['Database', 'Connection']"]
 ] = ContextVar("databases:open_connections", default=None)
-
 _ACTIVE_TRANSACTIONS: ContextVar[
-    typing.Optional[weakref.WeakKeyDictionary["Transaction", "TransactionBackend"]]
+    typing.Optional["weakref.WeakKeyDictionary['Transaction', 'TransactionBackend']"]
 ] = ContextVar("databases:open_transactions", default=None)