Right now we're only testing in CI that things work on macOS high sierra
but catalina has been out for a while and different osx versions differ
enough that it warrants testing that the new version works too. This
commit adds a new catalina job that runs only python 3.7 to verify that
things work on newer osx.

Co-Authored-By: Kevin Krsulich <[email protected]>

With the addition of python 3.8 jobs we're running at our job
concurrency limit. To avoid making the run time of ci too long by having
to wait on every commit this commit switches the second stage osx jobs
to use mojave instead of adding duplicated jobs on multiple osx
versions. We'll still use high sierra for the first stage's 3.7 testing
to maintain coverage there.

Catalina images have been available on azure pipelines for a few days
now. This commit pivots to using those instead of 10.14 mojave which
have issues with segfaults because of system libraries.

The pip installable numpy is causing segfaults on catalina and mojave
environments. To workaround this issue this commit switches to using a
conda install for the macos jobs only.

10.15 fails in the same way with or without conda, we've tried 10.14
without conda but not with it. This commit switches back to 10.14 now
that we've switched to using conda to see if the tests pass.

Now that we have a fix for the random segfaulting in the quantum info
tests there shouldn't be a reason we need to keep running in conda. At
the same time this switches back to 10.15 to test the newest version of
macOS. This only reverted back to 10.14 briefly to see if the segfault
was isolated to 10.15, which it was not.