Try the following on the Python command line:
import platform
platform.architecture()[0]
It should display '64bit' Just having a x86 version of Python is not enough. I have the same problem. Thought I had a 64-bit installation, but it turned out to be 32-bit.
BTW. it also works great with the Conda Python 3.6 distribution. Indeed, use the distro from the Gohlke page, as pointed out by Guillaume Jacquenot.