fix pip.req import error by not using pip internal modules

[cmek]

parse_requirements has been moved from pip.req to pip._internal so installing with newer versions of pip throws this error:

python setup.py install
Traceback (most recent call last):
File "setup.py", line 5, in
from pip.req import parse_requirements
ModuleNotFoundError: No module named 'pip.req'

[coveralls]

Coverage remained the same at 100.0% when pulling 762315e on cmek:pipreq_fix into d9fd5d3 on LINXNet:master.