read out class.__dict__ to avoid concurrent modification (#310)

Fix `RuntimeError: dictionary changed size during iteration` in
`ParserDispatchMixin._iter_class_members` when handling a class whose
`__dict__` is updated by `getattr`. Fix is to read out `__dict__` keys
into a list then iter. Resolves #309.

---------

Signed-off-by: htruscott <harrison@harriscott.net>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: htruscott

pybind11_stubgen/parser/mixins/parse.py

@@ -77,7 +77,8 @@ def _iter_class_members(self, class_: type):
         # Iterate __dict__ keys for definition order, but resolve values
         # through getattr() so descriptors (staticmethod, properties, etc.)
         # are properly unwrapped — matching inspect.getmembers() semantics.
-        for name in class_.__dict__:
+        names = list(class_.__dict__)
+        for name in names:
             seen.add(name)
             try:
                 value = getattr(class_, name)