Use unofficial Mutagen build to fix chapter times

[LSeelig]

This isn't the only chapter bug that might need a custom mutagen wheel, though I would hope that, if this is approved, another repo/host should be used.

[rdbende]

Suggested change

	"url": "https://github.com/LSeelig/mutagen/releases/download/release-1.47.1/mutagen-1.47.1-py3-none-any.whl",
	"sha256": "099c370f778f66b06f3e5a18e52948e53acdada530911c54090365b9292c99cb"
	}
	"url": "https://files.pythonhosted.org/packages/b0/7a/620f945b96be1f6ee357d211d5bf74ab1b7fe72a9f1525aafbfe3aee6875/mutagen-1.47.0-py3-none-any.whl",
	"sha256": "edd96f50c5907a9539d8e5bba7245f62c9f520aef333d13392a79a4f70aca719"
	},
	{
	"type": patch",
	"path": "some_patch_file_stored_in_cozy's_repo.patch"
	}

I'm not too keen on the idea of maintaining our own fork of Mutagen, and I see an easier way to implement fixes independently.
Flatpak manifests can patch specific submodules of a project, by applying a git-like patch file to files. This way we get all the new features in Mutagen, and can also fix bugs separately. The ultimate fix of course would be to fix these bugs upstream, but as I can see it, it's not really happening at the moment.