have stubs for minimal type annotation on addict

[LuisBL]

Today this minimal code:

$ cat /tmp/test.py
from addict import Dict

a = Dict()

can not be included in a fully typed project:

$ mypy /tmp/test.py
/tmp/test.py:1: error: Skipping analyzing "addict": module is installed, but missing library stubs or py.typed marker
/tmp/test.py:1: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)
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