Refactor SSLMode.parse so type checkers undertstand it

Author: DanielNoord

asyncpg/connect_utils.py

@@ -41,10 +41,10 @@ class SSLMode(enum.IntEnum):
     verify_full = 5
 
     @classmethod
-    def parse(cls, sslmode):
-        if isinstance(sslmode, cls):
-            return sslmode
-        return getattr(cls, sslmode.replace('-', '_'))
+    def parse(cls, sslmode: typing.Union[str, "SSLMode"]) -> "SSLMode":
+        if isinstance(sslmode, str):
+            return getattr(cls, sslmode.replace('-', '_'))
+        return sslmode
 
 
 class SSLNegotiation(compat.StrEnum):