Use environment markers to only install pycbc for python-2.7
This PR adds an environment marker [see PEP 508] to requirements-dev.txt
to only install pycbc for python 2.7. This should allow the python3 CI builds to complete.
This PR adds an environment marker [see PEP 508] to requirements-dev.txt
to only install pycbc for python 2.7. This should allow the python3 CI builds to complete.