Free threading support

[colesbury]

Packaging and testing

• Run pytest-run-parallel tests in CI
• Build wheels?
• Rename and edit metadata for fork?

Thread-safety (todo)

• Some bits in ct_flags are mutable, like CT_LAZY_FIELD_LIST, CT_IS_OPAQUE, and CT_CUSTOM_FIELD_POS | CT_WITH_PACKED_CHANGE. These may need to be moved to ct_flags_mut and we need synchronization for the handling of CT_LAZY_FIELD_LIST.
• b_complete_struct_or_union (related to above). This realizes the lazy field list.

Thread-safety (done)

• get_primitive_type: We initialize all primitive types at module initialization, instead of doing them lazily. This is pretty inexpensive (~100 µs).
• file_struct: now initialized during module initialization
• _get_ct_int: now initialized during module initialization
• init_once_cache: The cache itself is initialized under a critical section and we use APIs that return strong references instead of borrowed references.
• malloc_closure: use a global mutex
• ...