remove broken remnants of Python 2 compatibility

[a-detiste]

by using typing, which is a good thing,
this project cannot be Python2-compatible anymore;
this cleans this up

$ grep Union -r
markdownify/__init__.pyi:from typing import Callable, Union
markdownify/__init__.pyi:    code_language_callback: Union[Callable[[Incomplete], Union[str, None]], None] = ...,
markdownify/__init__.pyi:    convert: Union[list[str], None] = ...,
markdownify/__init__.pyi:    strip: Union[list[str], None] = ...,
markdownify/__init__.pyi:    strip_document: Union[str, None] = ...,
markdownify/__init__.pyi:    strip_pre: Union[str, None] = ...,
markdownify/__init__.pyi:    wrap_width: Union[int, None] = ...,
markdownify/__init__.pyi:        code_language_callback: Union[Callable[[Incomplete], Union[str, None]], None] = ...,
markdownify/__init__.pyi:        convert: Union[list[str], None] = ...,
markdownify/__init__.pyi:        strip: Union[list[str], None] = ...,
markdownify/__init__.pyi:        strip_document: Union[str, None] = ...,
markdownify/__init__.pyi:        strip_pre: Union[str, None] = ...,
markdownify/__init__.pyi:        wrap_width: Union[int, None] = ...,
tests/types.py:from typing import Union
tests/types.py:def callback(el: BeautifulSoup) -> Union[str, None]: