CI: macOS client builds done with new gitlab-runner beta

Gitlab has a new macOS runner service. We're in the beta. So resurrecting our attempts at using pyinstaller build son macOS.

TODO list

• pyinstaller installed
  • access with hardcoded path
• get pyinstaller working
  • looks like python-dev (as it would be on linux) is missing (?)
  • maybe https://github.com/danhper/asdf-python/issues/38
  • yes did that, basically install our own python.
• make a full test suite / server install on mac job: future MR!
• probably we want to keep just 10.13 and 11 (?)
  • 10.15 is often/always really slow at building python, like 20 mins when 10.13 is 5 mins.
• use requirements.txt.client instead of hardcoding deps in .gitlab-ci.yml

Images

I cannot find where it is documented about what images are available... I eventually found [1] which got me started. Later I found [2]. Currently this branch does:

image name	OOTB python
macos-10.13-xcode-9	python 2
macos-10.14-xcode-10	python 2
macos-10.15-xcode-11	python 3.8.0
macos-11-xcode-12	python 3.9.1

[1] OpenMW/openmw!690 (diffs) [2] https://gitlab.com/gitlab-org/ci-cd/shared-runners/images/macstadium/orka