Fixed `clean-python` Makefile target.
Fixed `clean-python` Makefile target.

Our usage of `find ... -exec` did not take into account the order of
operations in `find`.  The placement of `-exec` at the end meant that
only the `*.egg-info` files were being deleted.

Adding parentheses around all the `-o` operators fixes what is being
deleted.

Review: https://reviews.apache.org/r/50717/

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