stubtest: Fix `__mypy-replace` false positives (#15689)

mypy/stubtest.py

diff --git a/mypy/stubtest.py b/mypy/stubtest.py
index f06faa9..906a8c9 100644
--- a/mypy/stubtest.py
+++ b/mypy/stubtest.py
@@ -496,7 +496,11 @@
     )
 
     # Check everything already defined on the stub class itself (i.e. not inherited)
-    to_check = set(stub.names)
+    #
+    # Filter out non-identifier names, as these are (hopefully always?) whacky/fictional things
+    # (like __mypy-replace or __mypy-post_init, etc.) that don't exist at runtime,
+    # and exist purely for internal mypy reasons
+    to_check = {name for name in stub.names if name.isidentifier()}
     # Check all public things on the runtime class
     to_check.update(
         m for m in vars(runtime) if not is_probably_private(m) and m not in IGNORABLE_CLASS_DUNDERS