Fix case when theme is provided via an remote https source

[loechel]

Diazo brakes with an IOError due to lxml when the theme is from an remote https source.
Fallback to an urllib urlopen call to get the theme if it is https.

solevs #40

[lrowe]

I think this is what's causing the flake8 error. Diazo depends on the future module, so it should work to just import from the new location (may need to bump the minimum version for future in setup.py) http://python-future.org/standard_library_imports.html

[loechel]

well now it should work in general. If you accept it and merge it it would be nice.

[lrowe]

I've fixed the flake8 failures on master, but it looks like there are other errors coming up with this change. Sorry, I don't have time to look more into it now.

[loechel]

have looked into the problem and fixed, so that all test passes again.
We should write another explicit test for checking remote themes via varios protocols. If anybody can give me a hint, i would do that, but I don't understand the current test setup.