Conformance - document Python 3.11 requirement for pytype tests

[Andrej730]

Not sure if it's still required-required by pytype - I haven't investigated, but wanted to document as it is the current conformance requirement.

Source: https://github.com/python/typing/blob/0b9fed83ee2ac10aeac187d4182ae949f9fc25f0/conformance/src/type_checker.py#L313-L318