CI: Pin Python to 3.7.3
I'm currently looking to see what other projects are affected.   Maybe try master from 2 weeks ago? Oh, we're on python 3.7.4 now. That's most likely it. Let's get CI passing, and investigate later. The windows 3.7.4 build passed just fine, so I didn't pin that to 3.7.3. This may break the pip dep handling in https://github.com/pandas-dev/pandas/blob/master/scripts/generate_pip_deps_from_conda.py#L22. We skip exactly on `python=3` there.

But I'd like to have a single commit that we can revert when the conda 3.7.4 packages are sorted out. all green, LGTM