List all classes in a package directory
Since the classes are all subclasses of _MyBase, they can be accessed via _MyBase.__subclasses__() after they have been imported:

for (module_loader, name, ispkg) in pkgutil.iter_modules([pkg_dir]):
    importlib.import_module('.' + name, __package__)

all_my_base_classes = {cls.__name__: cls for cls in base._MyBase.__subclasses__()}


For importing the modules, I followed the advice of Nihathrael.