The reason this came up was because of mypy giving the error:

error: Invalid index type "Literal['WRITEABLE']" for "flagsobj"; expected type "Literal['A', 'ALIGNED', 'W', 'WRITABLE', 'X', 'WRITEBACKIFCOPY']"

on my code:

vertex.flags['WRITEABLE'] = False

Thanks @knappa, this seems consistent with the other *.pyi files. It also got me searching WRITABLE vs WRITEABLE, and, wouldn't you know it, cpython defines both PyBUF_WRITABLE and PyBUF_WRITEABLE with the latter an alias of the first. We use both versions in our code, that's just crazy :(