[3.6] bpo-31588: Validate return value of __prepare__() methods (GH-3790)

[3.6] bpo-31588: Validate return value of __prepare__() methods (GH-3790)

Class execution requires that __prepare__() methods return
a proper execution namespace. Check for that immediately
after calling __prepare__(), rather than passing it through
to the code execution machinery and potentially triggering
SystemError (in debug builds) or a cryptic TypeError
(in release builds).

Patch by Oren Milman.
(cherry picked from commit 5837d0418f47933b2e3c139bdee8a79c248a943c)