Literals

[Strilanc]

Authored by rafaelha in #1010 then tweaked due to 3.6 and 3.7 support being dropped

Small quality of life improvement: Literal type hints to enable IDE autocomplete.

I tried to find all cases where Literal type hints can be provided. In a few places the stubs already had Literal type hints, but without importing from typing import Literal they did not do anything for me. I've gone ahead and added

try:
    from typing import Literal
except ImportError:
    pass

to dev/gen_stim_stub_file.py.

For Python <=3.7 one could also consider importing Literal from typing_extensions, but it's probably better not to introduce an additional dependency.