Changes proposed in this pull request

• Import external Python modules and functions using "private" names (for example, _sys) so they don't get included as members of the cantera module.
  • The one exception to this is np, which is used so extensively that I didn't want to have to change it. This one is removed by a del in __init__.py.
• Test that the resulting Python module does not (directly) contain any such modules or functions

This is an alternative approach to the same goal as #1947. There are a couple of things I discovered while preparing this.

• Because Pylance / VS Code builds its list of members through static analysis rather than by actually importing the module, some clever tricks to dynamically edit the list of namespace/module members don't work. Here, I still end up with the VS Code autocomplete showing me that warnings, Path, np, and os are members of the cantera namespace, despite being deleted in __init__.py
• The list of members obtained using the dir function, Jupyter's autocomplete (after import) and the autocomplete of from cantera import ... in IPython are correctly limited.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• The pull request is ready for review