A variable under a conditional statement marked as Final should be handled like a constant

[tristanlatr]

This piece of code does is accepted by mypy, so the astbuilder.is_constant() function should be adjusted.

if sys.version_info > (3,10):
    v:Final = 1
else:
    v:Final = 2

Let's note that mypy complains for the following code, but pydoctor should not make this extra step and still mark it as constant imo.

if random.choice([True, False]):
    v:Final = 1
else:
    v:Final = 2