Qt requires python built with newer SDK for macOS

[psavery]

Currently, both the anaconda and conda-forge distributions of python for macOS are built with an old SDK (10.9 or 10.10) to ensure backward compatibility on old OS's. Normally, you can simply specify an older MACOSX_DEPLOYMENT_TARGET, but they are going the extra mile and using older SDKs as well. Unfortunately, this is causing problems for us.

Qt turns on and off certain settings depending on which OS X SDK was used to build the binary that is running it, and the latest versions of Qt require newer SDKs for the binary. In our case, python is the binary running it. Since the python binaries on anaconda and conda-forge were built with an older OSX SDK, this results in a very buggy Qt application. The most noticeable bug is that many widgets (including the canvas) that are programmatically modified during runtime (e.g., setValue) do not update until you do something like re-size or de-focus the application.

Our current fix for this is to upload our own built python binary for mac that can be installed via conda install -c HEXRD python=3.8. But it will be nice if we can figure out some kind of upstream fix at some point, so we don't have to maintain our own python binaries.

Might be fixed by: conda-forge/python-feedstock#371

Issues caused (at least in part) by this issue:
#160
#196
#426
#492

[psavery]

As of right now, the python from Anaconda is still built with too old of an SDK. But the python from conda-forge starting in python 3.8 works. Thus, on Mac, we have to ensure we we are using the python from conda-forge. The README has already been updated in #770. Closing this issue.