It looks like
cffi-1.11.0-cp36-cp36m-macosx_10_6_intel.whl was built using cpython 3.6.1, which is ABI-incompatible with 3.6.0. This means that if you have 3.6.0 on MacOS then doing
pip install cffi; python3.6 -c 'import cffi' gives
ImportError: dlopen(/[...]/python3.6/site-packages/_cffi_backend.cpython-36m-darwin.so, 2): Symbol not found: _PySlice_AdjustIndices
Real life example: https://travis-ci.org/python-trio/trio/jobs/279922407
The solution is to rebuild the wheel using 3.6.0 or 3.6.2. If you build with 3.6.0 or 3.6.2, then the resulting wheel will run everywhere, including on 3.6.1; the bug only happens when you build using 3.6.1 and then try to run on 3.6.0. See bpo-29943.