Conda MacOS CI failures

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I don't understand the MacOS failures.

From one of the the failing runs, there's something that looks weird, though it may be unrelated: https://github.com/python-control/Slycot/runs/6246110913?check_suite_focus=true#step:4:1970

Processing dependencies for slycot==0.4.0
Searching for slycot==0.4.0
Reading https://pypi.org/simple/slycot/
Downloading https://files.pythonhosted.org/packages/85/21/4e7110462f3529b2fbcff8a519b61bf64e0604b8fcbe9a07649c9bed9d7a/slycot-0.4.0.0.tar.gz#sha256=1d9921e9b04a5b9892870fd3481f7b08e6fa083a1a3848ad262819de19eb5e02

I assume this is a mistake -- we don't want to download slycot 0.4.0 from PyPI for testing, do we?

I don't follow the final error message; I guess it failed, but why?

Running slycot-0.4.0.0/setup.py -q bdist_egg --dist-dir /tmp/easy_install-w0mc9zxi/slycot-0.4.0.0/egg-dist-tmp-qcfmz0w8
    sys.exit(main())
  File "/usr/local/miniconda/envs/build-env/lib/python3.9/site-packages/conda_build/cli/main_build.py", line 488, in main
    execute(sys.argv[1:])
  File "/usr/local/miniconda/envs/build-env/lib/python3.9/site-packages/conda_build/cli/main_build.py", line 477, in execute
    outputs = api.build(args.recipe, post=args.post, test_run_post=args.test_run_post,
  File "/usr/local/miniconda/envs/build-env/lib/python3.9/site-packages/conda_build/api.py", line 186, in build
    return build_tree(
  File "/usr/local/miniconda/envs/build-env/lib/python3.9/site-packages/conda_build/build.py", line 3088, in build_tree
    packages_from_this = build(metadata, stats,
  File "/usr/local/miniconda/envs/build-env/lib/python3.9/site-packages/conda_build/build.py", line 2211, in build
    utils.check_call_env(cmd, env=env, rewrite_stdout_env=rewrite_env,
  File "/usr/local/miniconda/envs/build-env/lib/python3.9/site-packages/conda_build/utils.py", line 410, in check_call_env
    return _func_defaulting_env_to_os_environ('call', *popenargs, **kwargs)
  File "/usr/local/miniconda/envs/build-env/lib/python3.9/site-packages/conda_build/utils.py", line 390, in _func_defaulting_env_to_os_environ
    raise subprocess.CalledProcessError(proc.returncode, _args)
subprocess.CalledProcessError: Command '['/bin/bash', '-o', 'errexit', '/usr/local/miniconda/envs/build-env/conda-bld/slycot_1651398745184/work/conda_build.sh']' returned non-zero exit status 1.
Error: Process completed with exit code 1.

Originally posted by @roryyorke in #171 (comment)

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Could be that the current resolver is confused by the 0.4.0 vs 0.4.0.0.

Should try to fix this with #163 and probably #164