Register all public exports as public

[karloskalcium]

mutagen lists itself as py.typed however several of the modules are considered private - see this for more information.

example: mutagen.File, mutagen.MutagenError are both stored in "_xx" files which is considered private.

Fixing this will eliminate errors from type checkers. Thanks!

[DidierMalenfant]

I'm hitting this too.

Public types look to be:

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

Would it be ok to submit a PR with those 3 files renamed as util.py, file.py and tags.py?