[ENH] Pull YAML feed of notifications on startup, refactor notifications

[irgolic]

Should be merged after biolab/orange-canvas-core#15.

Issue

Likely the final pull request implementing #3372.

Description of changes

• Download and parse list of notifications in YAML format.
• Refactored notifications so as to expose a simpler interface to the programmer.
• Fix preferences access while parsing YAML (QSettings dictionary interface)

Includes

• Code changes
• Tests
• Documentation

[ales-erjavec]

Prefer a

   def __init__(self):
       raise TypeError(
           "`NotificationServer` is a singleton and cannot be instantiated. "
           "`Use NotificationServer.instance()` instead"
       )

    @classmethod
    def instance(cls) -> NotificationServer:
        """
        Return the single instance of the NotificationServer class.

        Returns
        -------
        instance: NotificationServer
        """
        if cls.__instance is None:
            cls.__instance = NotificationServer.__new__(NotificationServer)
            super(NotificationServer, cls.__instance).__init__()
        return cls.__instance

and use a NotificationServer.instance() to get the instance. It's clearer and does not need a metaclass (which is too heavy handed for this).

Does the NotificationServer actually need to be a Singleton. From what I can see it is (first) created in main() and is passed to the first main window from whence it propagates further to new windows. That only leaves thepull_notifications() in __main__ which could easily take the instance created in main() as a parameter.

It is not hard to envision multiple separate queues.

[ales-erjavec]

I seem to have missed #3933 (comment)

[markotoplak]

@ales-erjavec, I think that we could still avoid singletons. If an instance is created in main and simply attached to something (module?) that is accessible anywhere, it would still address my concern.

[irgolic]

Like so?

[codecov]

Codecov Report

Merging #3933 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3933   +/-   ##
=======================================
  Coverage   85.26%   85.26%           
=======================================
  Files         385      385           
  Lines       68762    68762           
=======================================
  Hits        58630    58630           
  Misses      10132    10132

[lanzagar]

While we (probably?) don't support PyQt4 anymore it would still be better to use AnyQt for imports...

[lanzagar]

Where is this used?

[lanzagar]

Found it: __main__.py L26