Add Literal type hints for IDE autocomplete

[rafaelha]

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.

[Strilanc]

I was waiting for 3.6 and 3.7 support to be dropped before merging this (as I don't like the types being invalid in some of the versions). Now that that's happening, I've extended this change to not use the try-catch import in #1010 . Thanks for the submission!