bpo-42333: Port _ssl extension to multiphase initialization (PEP 489) (GH-23253)

- Introduce sslmodule_slots
- Introduce sslmodulestate
- Use sslmodulestate
- Get rid of PyState_FindModule
- Move new structs and helpers to header file
- Use macros to access state
- Keep a strong ref to socket type

Author: tiran

Misc/NEWS.d/next/Library/2021-04-17-19-31-17.bpo-42333.cgbtZO.rst

@@ -0,0 +1 @@
+Port ``_ssl`` extension module to multiphase initialization.