Make the Python 2 and 3 concurrent.futures stubs identical so fixes get
applied to both.
For example, #1305 and #2233 fixed the same problem at different times,
as did #1078 and #1911.
By making the stubs identical, we apply the fix from #1711 to Python 2.
Fixes#2234.
The dict stub was referring to an instance, not the type, leading to
__call__ being considered when using as a decorator, rather than
__init__.
mock is a backport of the stdlib module and should be defined the same.