Add documentation the minimum suppported python version is 3.9

[tjoneslo]

The @functools.cache in the new FontLayer requires Python 3.9. So note in the documentation this is now the minimum required version for PyRoute.

Note, I may expand the use of this function into other places in the code as it may make specific internal processing to run faster.

[CyberiaResurrection]

@tjoneslo , time to close this?