Regular reST section headings do not work (see this fairly recent mailing list thread, and also this older thread), but the .. rubric::
directive can be used as a heading in docstrings. Perhaps you can use something like this:
class MyClass(object):
"""
.. rubric:: Class variables
:cvar foo: foo documentation
:cvar bar: bar documentation
.. rubric:: Instance variables
:ivar baz: baz documentation
"""
pass