Add __all__ to __init__.py (#696)

* Add __all__ to root __init__.py

* Add version, version_string to __all__ too, to not break API

---------

Co-authored-by: Christoph Reiter <reiter.christoph@gmail.com>

Author: ChadThackray

mutagen/__init__.py

@@ -28,16 +28,14 @@
 version_string = ".".join(map(str, version))
 """Version string."""
 
-MutagenError
-
-FileType
-
-StreamInfo
-
-File
-
-Tags
-
-Metadata
-
-PaddingInfo
+__all__ = [
+    "MutagenError",
+    "FileType",
+    "StreamInfo",
+    "File",
+    "Tags",
+    "Metadata",
+    "PaddingInfo",
+    "version",
+    "version_string",
+]