Add async utils from antsibull-core

[felixfontein]

Disadvantage of having this here: antsibull-changelog so far doesn't depend on aiofiles.

[gotmax23]

One idea is to make this a dependency-less package and have one extra per module. Each module/package can have something like

try:
    import module
    import other_module
except ImportError as exc:
    raise ImportError(f"{__package__} requires the hashing/io/whatever extra. Please install antsibull-fileutils[EXTRA_NAME]") from exc

On the other hand, aiofiles has no dependencies itself, and it doesn't compile any C extensions or anything, so perhaps not a big deal to add as a hard dependency.

[felixfontein]

After thinking some more about this, I'm for simply merging this and thus adding aiofiles as a dependency.

[felixfontein]

I've adjusted ansible-community/antsibull-core#166, ansible-community/antsibull-build#619, and ansible-community/antsibull-docs#322 to use the code from this PR.

[gotmax23]

After thinking some more about this, I'm for simply merging this and thus adding aiofiles as a dependency.

Sounds good to me!

[felixfontein]

@gotmax23 thanks for reviewing this!