[3.9] bpo-41194: Convert _ast extension to PEP 489 (GH-21807)

[3.9] bpo-41194: Convert _ast extension to PEP 489 (GH-21807)

* bpo-41194: Convert _ast extension to PEP 489 (GH-21293)

Convert the _ast extension module to PEP 489 "Multiphase
initialization". Replace the global _ast state with a module state.

(cherry picked from commit b1cc6ba73a51d5cc3aeb113b5e7378fb50a0e20a)

* bpo-41204: Fix compiler warning in ast_type_init() (GH-21307)

(cherry picked from commit 1f76453173267887ed05bb3783e862cb22365ae8)