Allow observing of monkey patched methods

[ckbaumann]

This PR allows to observe monkey patched methods (you replace or modify method attributes of a class instance s.t. methods are called that were not in the original class definition).

Additionally, the error when trying to add private methods as observers was made a little more explicit.

[papr]

@ckbaumann Starting the Head Pose Tracker in Capture using the --debug arg results in this traceback:

Traceback (most recent call last):
  File "/Users/papr/work/pupil/pupil_src/launchables/world.py", line 648, in world
    g_pool.plugins = Plugin_List(g_pool, loaded_plugins)
  File "/Users/papr/work/pupil/pupil_src/shared_modules/plugin.py", line 383, in __init__
    self.add(plugin, args)
  File "/Users/papr/work/pupil/pupil_src/shared_modules/plugin.py", line 398, in add
    plugin_instance = new_plugin_cls(self.g_pool, **args)
  File "/Users/papr/work/pupil/pupil_src/shared_modules/head_pose_tracker/online_head_pose_tracker.py", line 53, in __init__
    self._setup_renderers()
  File "/Users/papr/work/pupil/pupil_src/shared_modules/head_pose_tracker/online_head_pose_tracker.py", line 76, in _setup_renderers
    self._detection_renderer = plugin_ui.DetectionRenderer(
  File "/Users/papr/work/pupil/pupil_src/shared_modules/head_pose_tracker/ui/detection_renderer.py", line 41, in __init__
    plugin.add_observer("gl_display", self._on_gl_display)
  File "/Users/papr/work/pupil/pupil_src/shared_modules/observable.py", line 75, in add_observer
    add_observer(self, method_name, observer)
  File "/Users/papr/work/pupil/pupil_src/shared_modules/observable.py", line 135, in add_observer
    _install_protection_descriptor_if_not_exists(wrapper)
  File "/Users/papr/work/pupil/pupil_src/shared_modules/observable.py", line 172, in _install_protection_descriptor_if_not_exists
    wrapped_method = wrapper.get_wrapped_bound_method()
  File "/Users/papr/work/pupil/pupil_src/shared_modules/observable.py", line 297, in get_wrapped_bound_method
    return self._wrapped_method_ref.deref_method()
  File "/Users/papr/work/pupil/pupil_src/shared_modules/observable.py", line 421, in deref_method
    raise _ReferenceNoLongerValidError
observable._ReferenceNoLongerValidError

[papr]

@ckbaumann For reference, this is how we patch the gl_display method.

[papr]

@ckbaumann Works as expected now! Thanks for working on the fix!

[ckbaumann]

Rebased onto current develop.
@papr The line you applied black to was modified in the last commits, so I removed the "apply black" commit to keep history shorter
@papr needs review starting with 61604d3