You can find the new method names in Glyphs 3.app/Contents/Frameworks/GlyphsCore.framework/Versions/A/Headers/GSOutlineImporter.h
this:
- (BOOL)readFile:(NSURL *)URL toLayer:(GSLayer *)layer bounds:(NSRect *)outBounds error:(NSError **)error;
translates to python:
OutlineImporter.readFile_toLayer_bounds_error_(fileURL, layer, None, None)