Add __all__ to __init__.py

[ChadThackray]

Addresses #647.

When using mypy in strict mode I am forced to import items from root mutagen namespace as:

from mutagen import File as MutagenFile  # type: ignore[attr-defined]

If I remove the ignore statement I get this error:

src/shares/indexer.py:12: error: Module "mutagen" does not explicitly export attribute "File"  [attr-defined]

There is another PR relating to this issue: #695
But the tests are failing for that due to some changes made

[lazka]

Apparently this also makes mypy happy: https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-no-implicit-reexport

from mutagen._util import MutagenError as MutagenError
from mutagen._file import FileType as FileType, StreamInfo as StreamInfo, File as File
from mutagen._tags import Tags as Tags, Metadata as Metadata, PaddingInfo as PaddingInfo

not sure which one I prefer

[lazka]

I guess we use __all__ in various places, so let's go with that.

In other places were we don't want to change __all__ that's what we have to do though.

[scratchmex]

This could be co-authored but ok, that's why reviews exist. I'm not trying to be pendantic tho