[BUG] Regression on callbacks using name argument `output=Output(...)`

[Marc-Andre-Rivet]

Caused by #1180

Example DCC failing test: https://github.com/plotly/dash-core-components/blob/dev/tests/test_integration_1.py#L391

Removing (output=) makes it work:

Output(component_id='test-pathname', component_property='children'),

The problematic line seems to be https://github.com/plotly/dash/blob/dev/dash/dependencies.py#L163

Updating to 👇 makes the test pass:

if isinstance(outputs, (list, tuple)) and len(outputs) == 1:

Expected result:

• DCC tests pass
• Added test in Dash for named arguments

@alexcjohnson