stubgen cannot infer precise literal string types from docstring

Bug Report

I'm using stubgen to generate pyi files for a python c extension module.
Some functions take arguments of type Literal["xxx", "yyy"]; such constraints are written in the docstring of those functions.
After stubgen, these annotations are missing; however this only happens when the arguments of Literal contain str or bytes type.
This problem can be reproduced using infer_sig_from_docstring (see below).

To Reproduce

from mypy.stubdoc import infer_sig_from_docstring

# literal int and bool works as expected
print(infer_sig_from_docstring("f(x: Literal[42]) -> None", "f"))
print(infer_sig_from_docstring("f(x: Literal[False]) -> None", "f"))

# doesn't work
print(infer_sig_from_docstring("f(x: Literal['4_2']) -> None", "f"))
print(infer_sig_from_docstring('f(x: Literal["4_2"]) -> None', "f"))
print(infer_sig_from_docstring("f(x: Literal[b'4_2']) -> None", "f"))
print(infer_sig_from_docstring('f(x: Literal[b"4_2"]) -> None', "f"))

Expected Behavior

[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal[42]', default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal[False]', default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal["4_2"]', default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal["4_2"]', default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal[b"4_2"]', default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal[b"4_2"]', default=False)], ret_type='None')]

Actual Behavior

[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal[42]', default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type='Literal[False]', default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type=None, default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type=None, default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type=None, default=False)], ret_type='None')]
[FunctionSig(name='f', args=[ArgSig(name='x', type=None, default=False)], ret_type='None')]

Your Environment

• Mypy version used: 0.982
• Python version used: 3.10.8

Start with the reported infer_sig_from_docstring entry point and reproduce the six examples from the issue. Trace how Literal annotations are parsed, then add coverage for string and bytes values so the returned signatures preserve the expected Literal types.