Fix Python 3.8 Union type checking.

Author: FayeDel

discord_slash/model.py

@@ -483,13 +483,16 @@ def from_type(cls, t: type):
         if issubclass(t, discord.abc.Role):
             return cls.ROLE
         # Here's the issue. Typechecking for a **Union** somewhat differs per version (from 3.6.8+)
-        if (
-            hasattr(typing, "_GenericAlias")
-            and isinstance(t, typing._UnionGenericAlias)  # noqa
-            or not hasattr(typing, "_GenericAlias")
-            and isinstance(t, typing._Union)  # noqa
-        ):
-            return cls.MENTIONABLE
+
+        if hasattr(typing, "_GenericAlias"):  # 3.7 onwards
+            if isinstance(t, typing._UnionGenericAlias):  # noqa
+                return cls.MENTIONABLE  # 3.9+
+            elif t.__origin__ is typing.Union:  # 3.7-3.8
+                return cls.MENTIONABLE
+        else:  # py 3.6
+            if isinstance(t, typing._Union):  # noqa
+                return cls.MENTIONABLE
+
         if issubclass(t, float):
             return cls.FLOAT