Check version of cryptography module

We have had multiple issues with Ubuntu 16.04 using an outdated version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 Kodi startup so the user is aware
of the issue.

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

resources/language/English/strings.po

@@ -111,3 +111,17 @@ msgstr ""
 msgctxt "#32035"
 msgid "It is recommended that you to upgrade to a newer version."
 msgstr ""
+
+#empty strings from id 32036 to 32039
+
+msgctxt "#32040"
+msgid "Your version %s of the Python cryptography module is too old. You need at least version 1.7."
+msgstr ""
+
+msgctxt "#32041"
+msgid "Please upgrade your operating system."
+msgstr ""
+
+msgctxt "#32042"
+msgid "For more information, see https://kodi.wiki/view/Linux"
+msgstr ""

service.py

@@ -19,8 +19,9 @@
 
 import platform
 import xbmc
+import xbmcgui
 import lib.common
-from lib.common import log, dialog_yesno
+from lib.common import log, dialog_yesno, localise
 from lib.common import upgrade_message as _upgrademessage
 from lib.common import upgrade_message2 as _upgrademessage2
 
@@ -102,9 +103,27 @@ def _versionchecklinux(packages):
         log("Unsupported platform %s" %platform.dist()[0])
         sys.exit(0)
 
-
+# Python cryptography < 1.7 (still shipped with Ubuntu 16.04) has issues with
+# pyOpenSSL integration, leading to all sorts of weird bugs - check here to save
+# on some troubleshooting. This check may be removed in the future (when switching
+# to Python3?)
+# See /~https://github.com/pyca/pyopenssl/issues/542
+def _checkcryptography():
+    ver = None
+    try:
+        import cryptography
+        ver = cryptography.__version__
+    except:
+        # If the module is not found - no problem
+        return
+
+    ver_parts = list(map(int, ver.split('.')))
+    if len(ver_parts) < 2 or ver_parts[0] < 1 or (ver_parts[0] == 1 and ver_parts[1] < 7):
+        log('Python cryptography module version %s is too old, at least version 1.7 needed' % ver)
+        xbmcgui.Dialog().ok(ADDONNAME, localise(32040) % ver, localise(32041), localise(32042))
 
 if (__name__ == "__main__"):
+    _checkcryptography()
     if ADDON.getSetting("versioncheck_enable") == "false":
         log("Disabled")
     else: