…1705)

This fixes memory unsafety when there is a race condition related to
attribute get/set operations on a free-threaded build, for simple
reference-based attributes only (`PyObject *`). This includes attributes
with primitive types like `list`, `set` and `str` and `dict`, and native
class types.

The main idea is to use delayed decref for the old attribute value when
assigning to an attribute. There are detailed comments explaining the
approach in detail.

This causes a ~5% slowdown in self check when using 3.14t. Currently it
looks like a significant perf cost is unavoidable if we want to fix
memory unsafety, but we can further optimize mypy to reduce the impact
by introducing final attributes, for example.

Remaining work includes fixing attributes with fixed-length tuple types
and tagged integer types (in follow-up PRs).

I used coding agent assist heavily.

Work on mypyc/mypyc#1203.

…21726)

Mypyc now gives additional thread safety guarantees.

Release tracking issue: #21717