Work around setuptools 60 release in test. (#1548)
Work around setuptools 60 release in test. (#1548)

The release added a warning that trips up a test. Pin low to avoid this
since setuptools version is incidental to the test.

See: https://github.com/pypa/setuptools/blob/main/CHANGES.rst#v6000
pantsbuild/pex,jsirois/pex,jsirois/pex,pantsbuild/pex