bpo-40077: Convert _json module to use PyType_FromSpec() (GH-19177)

Replace statically allocated types with heap allocated types:
use PyType_FromSpec().

Add a module state to store the Scanner and Encoder types.
Add traverse, clear and free functions to the module.

Author: corona10

Misc/NEWS.d/next/Core and Builtins/2020-03-27-01-11-08.bpo-40077.wT002V.rst

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+Convert json module to use :c:func:`PyType_FromSpec`.