Annotate and document compatibility.py

[knutnergaard]

@benkiel b0efc71 results in the folllowing mypy errors:

glyph.py:2903: error: Argument 1 to "list" has incompatible type "set[tuple[Optional[str], int]]"; expected "Iterable[str]"  [arg-type]
glyph.py:2908: error: Argument 1 to "list" has incompatible type "set[tuple[Optional[str], int]]"; expected "Iterable[str]"  [arg-type]
Found 2 errors in 1 file (checked 24 source files)

This can be most easily solved by casting set[tuple[Optional[str], int]] to str, but I'm not sure if that's appropriate.

Also, should this module be documented?