[python] Check version of cryptography module

[yol]

We have had multiple issues with Ubuntu 16.04 using an outdated version
of the Python cryptography module that causes strange SSL behavior.
Example for this is not being able to connect to SSL sites.

This commit introduces a check on initialization of the interpreter
to make sure that we use a compatible version.

See also:
pyca/pyopenssl#542 (comment)
https://forum.kodi.tv/showthread.php?tid=335786

I just put the check inside the interpreter initialization - not sure if it's the most appropriate place? Also I don't have much experience with Python so please check.

How Has This Been Tested?

Build/Run on Linux x64

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

[wsnipex]

I don't think that failing to initialize python because of this is a good idea.
Just checked what pushing a fixed version to our PPA encompasses and - as expected - python-cryptography has several dependencies that need to either be bumped or don't exist at all in ubuntu 16.04

[yol]

What's the alternative really? It reproducibly breaks SSL (for some users at least?), meaning that even core functionality (such as installing add-ons) will not be available.

[wsnipex]

can we just log and throw up a gui error?

[yol]

Yes, but then we probably have to go about it differently - we'd need to do the check once at startup, or the user will get spammed all the time. I put it where it is now because there the interpreter is already initialized for sure. Any suggestions where we could place it otherwise? Or save a flag to only warn once?

Is it OK to warn the user once at startup? And do you mean like a toast-style notification or a full message box?

[yol]

@notspiff Do you have a suggestion how we could easily show a message box at startup? To use PythonInvoker, it seems I'd have to put the script into a file. Would it be possible/reasonable to make a small add-on that just checks the version once at startup?

[wsnipex]

maybe in the version check addon? https://github.com/xbmc/xbmc/tree/master/addons/service.xbmc.versioncheck

although it can be disabled, at least on a fresh install, it should be enabled.

[yol]

Although it's called versioncheck, it's doing something completely different (check Kodi version). I'm not sure whether putting that check in there alongside would be a good idea.

@MartijnKaijser ?

[MartijnKaijser]

I'm fine with adding it

[yol]

Superseded by XBMC-Addons/service.xbmc.versioncheck#18