well pex is scrubbing site-packages and fails to find pytest:

pex: Unresolved requirements:
                     pex:   - pytest
                     pex:   - pytest-cov
                     pex:   - unittest2
                     pex:   - pytest-timeout
                     pex: Distributions contained within this pex:
                     pex:   None
                     Failed to execute PEX file, missing compatible dependencies for:
                     pytest
                     pytest-cov
                     unittest2
                     pytest-timeout