Improve dependencies compatibility definition, and testing

[springload]

Improve dependencies compatibility definition, and testing

[loicteixeira]

I reckon it's safer to have an upper bound for both for now as we can always increase it later once we confirm everything is working rather than having to rush a patch.

beautifulsoup4>=4.4.1,<5 and lxml>=3.6.0,<4 would be a good start.

If you're agree with that I'll update the test-matrix and documentation when I have a moment.

Edit: I realize now that the name of the package beautifulsoup4 kinda implies that there will never be a version 5 and that they would instead release a separate beautifulsoup5 package but we can't rely solely on assumptions.

[thibaudcolas]

Sounds good!

[thibaudcolas]

lxml 4.0.0 has been released in september and I can confirm 4.1.1 works fine, so I'd suggest going for lxml>=3.6.0,<5.