Threading module has currentThread on python3

[jackriccomini]

Stub is defined like so:

if sys.version_info < (3,):
def currentThread() -> Thread: ...

But it seems Python 3.6 also has currentThread (although undocumented):

$ python3
Python 3.6.6 (default, Sep 12 2018, 18:26:19) 
[GCC 8.0.1 20180414 (experimental) [trunk revision 259383]] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import threading
>>> threading.currentThread
<function current_thread at 0x7febeb087840>

This issue is similar to #2552