I'm looking at adding typing to some parts of the code base (currently: pens) to make them nicer to work with in IDEs and mypy.

Example: looking at AbstractPointPen.addPoint, the segmentType can be one of a few strings or None. Python's typing.Literal covers that, but was added only in Python 3.8. For older Pythons, there's a backport package at https://pypi.org/project/typing-extensions/ that contains that type.

Another example is Protocol, which can be used to declare that a class implements some interface, but without necessarily deriving from some common base class. I suppose pens fit that description, but I'm not sure if there is some isinstance shenanigans going on somewhere outside fontTools?

Long story short, is it ok to add typing-extensions as a conditional dependency for Python < 3.8?