Fix PyPy handling in CalledFromGeneratedFile

[xylar]

The bypassing for PyPy got broken in https://github.com/protocolbuffers/protobuf/pull/10403

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[xylar]

I was informed in https://github.com/protocolbuffers/protobuf/issues/10305#issuecomment-1292480567 that this code may be (or may soon be) obsolete but it still seems to be getting built when we use --cpp_implementation, e.g. in this pull request: https://github.com/conda-forge/protobuf-feedstock/pull/167