Explicit Orange3 import

[woutdenolf]

I have an installation which only has orange-canvas an orange-base-widget and it complains about Orange3 not being installed

----------------------- DistributionNotFound Exception ------------------------
Traceback (most recent call last):
  File "/home/denolf/dev/orange-canvas-core/orangecanvas/application/canvasmain.py", line 1901, in browse_examples
    if self.examples_dialog() == QDialog.Accepted:
  File "/home/denolf/dev/orange-canvas-core/orangecanvas/application/canvasmain.py", line 1846, in examples_dialog
    tutors = examples.workflows(config.default)
  File "/home/denolf/dev/orange-canvas-core/orangecanvas/application/examples.py", line 46, in workflows
    for ep in examples_entry_points():
  File "/home/denolf/dev/orange-widget-base/orangewidget/workflow/config.py", line 81, in examples_entry_points
    dist=pkg_resources.get_distribution("Orange3"))
  File "/users/denolf/virtualenvs/ewoks/ubuntu_20_04/py38/lib/python3.8/site-packages/pkg_resources/__init__.py", line 471, in get_distribution
    dist = get_provider(dist)
  File "/users/denolf/virtualenvs/ewoks/ubuntu_20_04/py38/lib/python3.8/site-packages/pkg_resources/__init__.py", line 347, in get_provider
    return working_set.find(moduleOrReq) or require(str(moduleOrReq))[0]
  File "/users/denolf/virtualenvs/ewoks/ubuntu_20_04/py38/lib/python3.8/site-packages/pkg_resources/__init__.py", line 891, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/users/denolf/virtualenvs/ewoks/ubuntu_20_04/py38/lib/python3.8/site-packages/pkg_resources/__init__.py", line 777, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'Orange3' distribution was not found and is required by the application
-------------------------------------------------------------------------------

So I guess the best way is to capture pkg_resources.DistributionNotFound and return an empty list here:

orange-widget-base/orangewidget/workflow/config.py

Lines 79 to 81 in 07416f7

	default_ep = pkg_resources.EntryPoint(
	"Orange3", "Orange.canvas.workflows",
	dist=pkg_resources.get_distribution("Orange3"))