Improve dependencies compatibility definition, and testing

[thibaudcolas]

Raised by @loicteixeira in https://github.com/springload/draftjs_exporter/pull/74/files#r135967185. Opening as a separate issue because it's worth discussing and there is no point in holding that PR for this.

At the moment, draftjs_exporter defines its dependencies as:

draftjs_exporter/setup.py

Lines 15 to 22 in c963514

	dependencies = [
	'beautifulsoup4>=4.4.1,<5',
	'html5lib>=0.999,<=1.0b10',
	]
	lxml_dependencies = [
	'lxml>=3.6.0',
	]

Those ranges are purposefully big (we want to support as many versions as possible for something as fundamental to people's tech stacks), which is good, but as @loicteixeira puts it then it would make sense to test accordingly, with the lower and upper bounds at least.

---

More info to help in the decision,

• lxml at 3.6.0 is the version I used locally. Seems like now there is 3.8.0 (https://pypi.python.org/pypi/lxml), might make sense to add an upper bound in setup.py to limit installed versions to below 4.0.0?
• The BeautifulSoup4 / html5lib versions are loosely aligned with requirements from Wagtail: https://github.com/wagtail/wagtail/blob/a7820a2e903a560d6ceb73062ab9ba922f23b0dc/setup.py#L30-L31

    "beautifulsoup4>=4.5.1",
    "html5lib>=0.999,<1",

Finally, bear in mind that our usage of the APIs of those dependencies is very small (HTML string -> DOM nodes conversion, DOM nodes -> HTML string conversion, create nodes, append child to node), which means that the potential breakage would only likely be in how those engines handle specific content, which is hard to test for. We do however have a small test suite of "potential engine quirks": https://github.com/springload/draftjs_exporter/blob/master/tests/engines/test_engines_differences.py

[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.