bpo-1635741: Port _thread to multiphase init (GH-23811)

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.

Author: vstinner

Misc/NEWS.d/next/Library/2020-12-16-23-28-52.bpo-1635741.Quy3zn.rst

@@ -0,0 +1,2 @@
+Port the :mod:`_thread` extension module to the multiphase initialization
+API (:pep:`489`) and convert its static types to heap types.