bpo-1635741: Convert _multibytecodec to multi-phase init (GH-24095)

Convert the _multibytecodec extension module (CJK codecs) to multi-phase
initialization (PEP 489).

Author: Erlend Egeberg Aasland

Misc/NEWS.d/next/Library/2021-01-04-15-05-40.bpo-1635741.EOCfZY.rst

@@ -0,0 +1,2 @@
+Convert the _multibytecodec extension module (CJK codecs) to multi-phase
+initialization (:pep:`489`).  Patch by Erlend E. Aasland.