Port the _thread extension module to the multiphase initialization
API (PEP 489) and convert its static types to heap types.

vstinner changed the title [WIP] bpo-1635741: Port _thread to multiphase init bpo-1635741: Port _thread to multiphase init

adorilson pushed a commit to adorilson/cpython that referenced this pull request

Port the _thread extension module to the multiphase initialization
API (PEP 489) and convert its static types to heap types.
Add a traverse function to the lock type, so the garbage collector
can break reference cycles.