Build with the latest SDK on macOS

Some projects require newer versions of the OSX SDK to be used, even
though the deployment target may be older. [Qt is one of those cases](https://doc.qt.io/qt-5/macos.html),
and using an older version of the OSX SDK to build Python, for instance,
[is causing some bugs when used with PySide2](https://github.com/conda-forge/conda-forge-ci-setup-feedstock/issues/67).

Can we use the latest OSX SDK when building the python binaries?

The ability to do this was added recently in conda-forge/conda-forge-ci-setup-feedstock#106. I think we would just need to add:
```
WITH_LATEST_OSX_SDK
- '1'
```

to the CI yaml files, such as [this one](https://github.com/conda-forge/python-feedstock/blob/master/.ci_support/osx_python3.8.____cpythontarget_platformosx-64.yaml).

Cross reference:
conda-forge/conda-forge-ci-setup-feedstock#67
conda-forge/pyside2-feedstock#46
ContinuumIO/anaconda-issues#11297
https://bugreports.qt.io/browse/PYSIDE-1083