Fix the int.__pow__ plugin

A recent typeshed change added a modulo argument to int.__pow__
which broke the plugin.

Tested manually. I didn't add a test because to be useful it would
need to be a cmdline test (so it uses the real typeshed) and this
doesn't seem worth adding a slow cmdline test for. Happy to add it if
someone disagrees though.

Fixes #7621.

Author: msullivan

mypy/plugins/default.py

@@ -365,8 +365,10 @@ def typed_dict_update_signature_callback(ctx: MethodSigContext) -> CallableType:
 
 def int_pow_callback(ctx: MethodContext) -> Type:
     """Infer a more precise return type for int.__pow__."""
-    if (len(ctx.arg_types) == 1
-            and len(ctx.arg_types[0]) == 1):
+    # int.__pow__ has an optional modulo argument,
+    # so we expect 2 argument positions
+    if (len(ctx.arg_types) == 2
+            and len(ctx.arg_types[0]) == 1 and len(ctx.arg_types[1]) == 0):
         arg = ctx.args[0][0]
         if isinstance(arg, IntExpr):
             exponent = arg.value