Use version ranges instead of pinning exact versions

[software-opal]

I'm trying to use your library on Python 3 and with Sphinx 2.0.1, and I have noticed that your package pins every package it depends on, this makes it very difficult to use new Sphinx versions.

[pauleikis]

Same issue, cannot use library because it's requirements are too strict

[clayrisser]

I am sorry about that. I built the requirements.txt file by running pip3 freeze > requirements.txt.

@software-opal would you be interested in removing the packages it does not depend on and submitting it as a pull request?

[clayrisser]

This should be fixed with version 0.5.0.