docs: always use dev_requirements.txt together with main version constraints in setup.py pip isn't good at considering old constraints when installing or updating packages later. Instead, install everything at once. Note: pip and setuptools are usually installed automatically and could perhaps be removed from documentation. But for now, let's keep it.

pytest >= 3.3.0, < 4.7
pytest-runner < 5.2
pytest-sugar >= 0.7.0, < 0.10
pytest-benchmark < 3.3
pytest-localserver < 0.6
mock < 3.1
Sphinx < 1.9
WebTest < 2.1
isort == 4.3.21