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]

[Jordandev678]

With respect to the primer results those errors seem fine.

 pp = dict(
        fft_mode=get_args(FFT_MODE_TYPE),
        mfft=[None, n, n+1, n+2],
        scaling=[None, 'magnitude', 'psd'],
        phase_shift=[None, -n+1, 0, n // 2, n-1])
    for f_typ, mfft, scaling, phase_shift in product(*pp.values()):
        if f_typ == 'onesided2X' and scaling is None:
            continue  # this combination is forbidden
        SFT = ShortTimeFFT(w, h_n, fs=n, fft_mode=f_typ, mfft=mfft,
                           scale_to=scaling, phase_shift=phase_shift)

#<snip from ShortTimeFFT definition>
   _fft_mode: FFT_MODE_TYPE = 'onesided'  # Mode of FFT to use

It's complaining fft_mode=f_typ isn't FFT_MODE_TYPE. By the time the get_args call makes it through the dict, values, and product calls to generate the test setups the result is str | int | None.

As for the other three related additions - they were only absent prior because before this get_args would have returned Any and with the Any in the mix it wouldn't have been type checked at all.

Worth someone else taking a look at this now.