Traits stubs complains about untyped containers

[corranwebster]

In the following example:

from traits.api import Dict, HasTraits, List, Set

class Test(HasTraits):
    example_list = List()
    example_dict = Dict()
    example_set = Set()

Mypy gives the following errors:

test_list_stub.py:6: error: Need type annotation for 'example_list'
test_list_stub.py:8: error: Need type annotation for 'example_dict'
test_list_stub.py:10: error: Need type annotation for 'example_set'
Found 3 errors in 1 file (checked 1 source file)

These are legitimate uses (equivalent to List(Any), etc.) but I don't think we want people to have to add the explicit Any, particularly given that Dict(Str) works (which is equivalent to Dict(Str, Any)).