Resolves #1188.

The change is simply to store a list, rather than a generator expression.

The new models_init.py.jinja template in the end-to-end tests shows that this now works. Without the fix in openapi.py, this template is rendered with nothing under the Using "openapi.models" and Using "openapi.enums" headings.

This fix is somewhat important for our use case, because we need to put some code in models/__init__.py that is aware of what the model class names and modules are. Without the fix, the only way to get this would be to iterate over imports, which gives you strings like from .module_name import ClassName, and then parse those strings, which is workable but pretty clunky.