Pinned semantic-version causes conflicts

[ulope]

A while ago semantic-version was pinned to <2.7. This is now starting to cause problems as other packages are moving to require newer versions (in our case py-solc-x which requires semantic_version>=2.8.1,<3).

I see that there's #86 (and the reworked branch based on that). Would it help for someone else to pick that up, or do you already know what needs to be done?

[bitprophet]

@ulope I haven't had a chance to return to that ticket in the ensuing year (sob) so if you have the time to take a crack at however it was left off, feel free.

Also, if it's relevant (may or may not be) I'll be dropping Python 2 support across my projects in the near future, so if supporting anything Py2 related would cause grief, just skip it. I'd be marking such a release as backwards incompatible as well (speaking of semver...) so if the least-bad way to handle newer semantic_version APIs involves NOT supporting versions of it <2.7, that's probably acceptable too.

Thanks!