simple workaround for package names containing dashes

[umeboshi2]

Using pip 19.2.3, the package names contain dashes, rather than underscores. To reproduce bug:

Create new virtual environment.

pip install -U pip
pip install pyramid-jwt
pip install django-rest-auth
pip freeze > reqs.txt
pip-upgrade reqs.txt | grep error

The PR fixes this problem, but also raises a runtime error which may not be desired.

[coveralls]

Coverage decreased (-1.2%) to 98.529% when pulling a16e3d7629ce7771fff791643bc76aa60df0d62a on umeboshi2:dashes-in-pkg-names into 5e2fb4c6f8294f8b8c89189547efd7f52b34b2ff on simion:master.

[simion]

A test for 100% coverage would be nice :)

[umeboshi2]

Tests fail when an index url is assigned in a local pip.conf file.