We have run out of flags bits in `PyTypeObject`'s `tp_flags`

PyTypeObject's tp_flags field is defined as an unsigned long. In effect this means that we have only 32 bits we can use for flags. We have already used all 32 of them.

Bits 15 and 16 are reserved for Stackless Python. Since Stackless Python is pining for the fjords, we could recycle those two bits. However that's not going to last long with likely changes necessary for parallelism and performance.

Any change to PyTypeObject breaks both API and ABI, but that's OK because:

• Adding a field or changing the size of a field is a backwards compatible API change.
• The existence of PyTypeObject is part of the stable ABI, but its layout is not.

The cleanest change, IMO, is to change unsigned long tp_flags to uint64_t tp_flags which gives us another 32 bits, which should last many more years.

先从 PyTypeObject 的定义及其 tp_flags 字段的所有使用处开始，然后根据所述的 API 和 ABI 约束评估拟议的宽度变更。完成的标准是达成并实现一项关于扩展可用 flag 位的共识设计，并处理受影响的接口和兼容性行为。