maestral: figure out how to set up dependencies on older systems so that it works

[barracuda156]

Just running it with a command fails:

36-67% maestral link
2024-04-10 17:51:44,107 - [bugsnag] WARNING - No API key configured, couldn't notify
Traceback (most recent call last):
  File "/opt/local/bin/maestral", line 8, in <module>
    sys.exit(main())
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/click/core.py", line 1157, in __call__
    return self.main(*args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/click/core.py", line 1688, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/maestral/cli.py", line 226, in wrapper
    return func(*args, **kwargs)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/maestral/cli.py", line 720, in link
    with MaestralProxy(config_name, fallback=True) as m:
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/maestral/daemon.py", line 693, in __init__
    from .main import Maestral
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/maestral/main.py", line 32, in <module>
    from .client import CONNECTION_ERRORS, DropboxClient, convert_api_errors
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/maestral/client.py", line 44, in <module>
    from .oauth import OAuth2Session
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/maestral/oauth.py", line 14, in <module>
    import keyring.backends.OS_X  # type: ignore
ModuleNotFoundError: No module named 'keyring.backends.OS_X'