Add assertion to help debugging typeshed issues (#9982)

JukkaL · web-flow · commit 844960680c1d · 2021-01-27T16:52:34.000Z
In some cases the VERSIONS file can't be found, and it's kind of hard to debug.
diff --git a/mypy/modulefinder.py b/mypy/modulefinder.py
@@ -692,6 +692,7 @@ def load_stdlib_py_versions(custom_typeshed_dir: Optional[str]) -> Dict[str, Tup
     result = {}
 
     versions_path = os.path.join(stdlib_dir, "VERSIONS")
+    assert os.path.isfile(versions_path), (custom_typeshed_dir, versions_path, __file__)
     with open(versions_path) as f:
         for line in f:
             line = line.strip()
