🐛 fix(docstring): survive a re-entrant docstring handler

[gaborbernat]

Building the docs died with ExtensionError: Handler <function process_docstring ...> threw an exception (exception: 'Config' object has no attribute '_annotation_globals'), reported in #750 against 3.5-3.6. process_docstring hangs three values off the config for the formatting code to read and deletes them in a finally. That works until documenting one object starts documenting another before the first one finishes: the inner call deletes the attributes, and the outer call then raises on its own teardown, which is the error the reporter saw. On their version an autodoc directive sitting in a docstring was enough, because the throwaway parse that places :rtype: ran it. The reproducer below builds clean since #624 neutralized those directives.

A typehints_formatter that documents a nested object still re-enters the handler on main, and still aborts the build the same way. Rather than block re-entry, the teardown now puts back whatever it found: an inner call restores the outer call's values, and the outermost call clears them. Nesting turns into an ordinary save and restore, whatever the caller did to reach it. 🔁

# conf.py
def setup(app):
    import demo
    from sphinx_autodoc_typehints import process_docstring

    def formatter(annotation, config):
        if annotation is int:
            process_docstring(app, "function", "demo.helper", demo.helper, None, ["Help."])
        return None

    app.config.typehints_formatter = formatter