Add special handling for typing.get_args

[Jordandev678]

This PR adds special handling for typing.get_args when the input is a Union[Literal...] allowing it to return that type instead of the current behavior of returning Any. Adds the handling and related test cases for get_args with varying types.

This allows mypy to understand when it is used as a type guard.

from typing import Literal, get_args
type_alpha = Literal["a", "b", "c"]
strIn: str = "c"
if strIn in get_args(type_alpha):
    reveal_type(strIn)  # N: Revealed type is "Union[Literal['a'], Literal['b'], Literal['c']]"
else:
    reveal_type(strIn)  # N: Revealed type is "builtins.str"

Fixes #15106

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy (https://github.com/scipy/scipy)
+ scipy/signal/tests/test_short_time_fft.py:419: error: Argument "fft_mode" to "ShortTimeFFT" has incompatible type "str | int | None"; expected "Literal['twosided', 'centered', 'onesided', 'onesided2X']"  [arg-type]
+ scipy/signal/tests/test_short_time_fft.py:419: error: Argument "mfft" to "ShortTimeFFT" has incompatible type "str | int | None"; expected "int | None"  [arg-type]
+ scipy/signal/tests/test_short_time_fft.py:420: error: Argument "scale_to" to "ShortTimeFFT" has incompatible type "str | int | None"; expected "Literal['magnitude', 'psd'] | None"  [arg-type]
+ scipy/signal/tests/test_short_time_fft.py:420: error: Argument "phase_shift" to "ShortTimeFFT" has incompatible type "str | int | None"; expected "int | None"  [arg-type]