Update dependencies

[D3X]

This PR:

• Adds support for pytest 5 and 6
• Adds support for python 3.7 and 3.8
• Drops support for pytest-selenium < 2
• Drops support for python < 3.6; python 2 is EOL and 3.5 is due to be EOL on 2020-09-13
• Moves from pytest-pep8 to pytest-pycodestyle; pep8 was renamed to pycodestyle a few years ago and pytest-pep8 is not compatible with pytest 6.
• Removes custom marks (page, element etc.) from tests. Custom marks have to be registered as of pytest 5 and it did not feel like a good idea to have each test marked with a separate custom mark. One can use pytest -k to match specific tests

[D3X]

@jlane9 Tests pass locally for me. Unfortunately, the travis build fails because of missing BrowserStack config.

[indris]

@D3X Hi, thank you for the update! I wrote an email to John. I hope maybe it can speed up PR approval a little