add test file for new 3.12 syntax

Author: asottile

testsuite/python312.py

@@ -7,3 +7,23 @@ def foo():
 
 def bar():
     pass
+#: Okay
+# new type aliases
+type X = int | str
+type Y[T] = list[T]
+type Z[T: str] = list[T]
+#: Okay
+# new generics
+def f[T](x: T) -> T:
+    pass
+
+
+def g[T: str, U: int](x: T, y: U) -> dict[T, U]:
+    pass
+#: Okay
+# new nested f-strings
+f'{
+    thing
+} {f'{other} {thing}'}'
+#: E201:1:4 E202:1:17
+f'{ an_error_now }'