If `PYTHONPATH` is set and points to the build location of the python
bindings package then when stubgen runs, `_mlir` will get imported twice
and bad things will happen (e.g., `Assertion !instance && “PyGlobals
already constructed”’`). This happens because `mlir` is a namespace
package and the importer/loader can't distinguish between
`mlir._mlir_libs._mlir` and `_mlir_libs._mlir` imported from `CWD`. Or
something like that. The fix is to filter out any entries in
`PYTHONPATH` that end in `MLIR_BINDINGS_PYTHON_INSTALL_PREFIX/..` (e.g.,
`python_packages/mlir_core/`).
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