Update dependencies

- add support for Python 3.7
- remove requirements.txt
- add dependency versions to setup.py
- remove Sphinx as main dependency; move to extras_required instead
6 jobs for master in 2 minutes and 16 seconds
Status Job ID Name Coverage
  Test
passed #125167732
python 3.4

00:00:58

passed #125167731
python 3.5

00:00:51

passed #125167730
python 3.6

00:00:55

passed #125167729
python 3.7

00:01:07

 
  Deploy
passed #125167733
pages

00:01:08

passed pages:deploy

00:00:01