Autodoc generates reST code that uses raw `:obj:` and `:class:` xrefs to
refer the classes and types. But they're fragile because they assume
the primary_domain=='py'.
This adds `:py:` prefix to these xrefs to make them robust.
Autoattribute directive should check the existence of instance attribute
that is defined inside __init__() but not having comments before
accessing it.
autodata and autoattribute directives does not show right-hand value of
the variable if its docstring contains ``:meta hide-value:`` in
info-field-list.
So far, autoattribute uses "given class name" to fetch comments of
uninitialized instance attributes. But pycode expects to use "real"
class name on searching attribute comments.
This adds UninitializedInstanceAttributeMixin to handle it simply.
