Extended R.H.S. on annotated assignments should imply Python 3.8+

[isidentical]

Before 3.8 (through 3.6-3.7) the annotated assignment's R.H.S. value can be only simple test: a: int = 1 or a: Tuple[int, ...] = (1, 2). Starting with 3.8, it now accepts a: Tuple[int, ...] = 1, 2 or even a: Tuple[int, ...] = b, *c, d