I fixed this using Anaconda by going to Environments > base(root), searching for numpy in the installed modules and clicking the tickbox alongside it and choosing > Mark for specific version installation > 1.14.0 (as suggested by another user on this thread). Then clicking Apply. Once it downgraded numpy I stopped getting errors when running py files on the command line.
Throughout this saga, I was still able to use https://pypi.org/project/auto-py-to-exe/ even when I was getting the numpy errors on the command line, but it was a hassle to create an exe every time I wanted to test a change. It's all sorted now. I guess there was a problem with numpy 1.16.4.
Anyway, I hope this helps someone who's using Anaconda as well.