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OK, is supporting this really necessary? It's a special case on Windows only; even OS X which is also case-insensitive doesn't support this.
But if it does need to be supported, then does someone know if this extends to all module types or only .py and .pyw files (e.g. do .pyc files need this along with extension modules)? If there is no special-casing to that extent then _PathFinder.find_module() will need to cache all files with lowercase file extensions no matter what PYTHONCASEOK says on Windows. That way the performance is only costly at cache building time and there is no expensive checking for all other OSs.