Python: add type annotations to all examples and documentation

[lgritz]

Our minimum supported python version is 3.7 currently (soon to be 3.9, I suspect). Python 2.x is long gone for us, but lots of our python examples still haven't embraced the new syntax with type annotations that has been available since python 3.5.

There is PR #4692 in progress now to add stub files, and in the process, adds type annotations to most of the testsuite python-*/src/test_*.py files.

But that still leaves:

• All inline python examples in the docs themselves src/doc/*.md (note: rendered docs at https://docs.openimageio.org)
• The documentation examples that have been converted into tests and incorporated into docs by reference: testsuite/docs-examples-python/src/docs-examples-*.py
• Any other auxiliary scripts that we have, such as testsuite/runtest.py

This would be a great Dev Days project! (Especially for somebody who wants to do something for OIIO but knows more Python than C++ and maybe didn't think they had as much to offer.) I don't think any one person needs to do it ALL, this could be the project of multiple people, as long as you divide up the work -- each person can claim a few particular files, or a particular chapter of the docs, and just do that while others pick different non-conflicting parts.

[lgritz]

Above, I cited some places where python code can be found in the project that might need this treatment. I did not look over it all myself. Maybe some chapters in the docs or scripts have lots of python functions that could be type annotated, maybe other chapters just contain a few python snippets that don't really have a lot of actual functions that should be annotated.

I don't know if this is one person's work for Dev Days, or is three people's worth of work. So if you want to do it, look over those areas of the code base, figure out which set of files look like what you want to bite off for DD, and comment here saying specifically which parts you will do, so that others know that they can pick up other pieces of it to work on if they want.