packaging: Add PyInstaller

[kinow]

Describe exactly what you would like to see in an upcoming release

This could be required in some sites due to I/O limitations, or other requirements. PyInstaller can package everything as a single file.

Additional context

There is some existing work on getting setuptools to work with PyInstaller:

• https://github.com/pyinstaller/pyinstaller/wiki/Recipe-Setuptools-Entry-Point

If nothing goes wrong, it could be a 1-day job to create a PyInstaller package with what we already have. But I think this is low priority for now :+1:

[kinow]

Learned about PyInstaller from the Improver developers. So if anybody needs to work on this, it might be a good idea to drop a message and see what we can learn from their work.

[matthewrmshin]

We'll most likely have to do this as part of Cylc-8 - as we have similar I/O issue and restricted network issue.