fix:Compatible with Python 3.7.x

[vchenpeng]

Before 5.x and 6.0 can run normally in python 3.7 environment, but pywebview v6.1 starts because it uses python 3.8 version specific syntax. The original code is as follows. In python 3.8 environment below, the error is reported.

The minimum requirement for the get_args package is Python 3.8+.

from typing import Any, Callable, cast, get_args

from typing_extensions import Literal, TypeAlias

to

from typing import Any, Callable, cast

from typing_extensions import Literal, TypeAlias, get_args