Support complex __all__ assignments

[pawamoy]

Is your feature request related to a problem? Please describe.
We claim to support __all__ variables for explicit exports. But we only do when it is assigned using a list/tuple and literal strings, not when summing lists and/or adding items later, and/or

Describe the solution you'd like
Support more complex assignments. Starred list elements and additions at least (+ and +=).

Describe alternatives you've considered
Stop reyling on __all__ and always resolve implicitly.

Additional context
Example: https://github.com/django/django/blob/main/django/db/models/__init__.py

[tristanlatr]

Looks like it's pretty easy with astroid to do that. Work with pydocspec currently.

The inference of the __all__ variable: https://github.com/tristanlatr/pydocspec/blob/main/pydocspec/processor/mod_attr.py#L13-L46 (where it's called: https://github.com/tristanlatr/pydocspec/blob/main/pydocspec/astbuilder.py#L246)

The test case: https://github.com/tristanlatr/pydocspec/blob/main/pydocspec/test/test_astbuilder.py#L435-L470

It does not understand list.extend and list.append but I think it would be pretty easy to add since it supports the augmented assigment already.