Deprecate old APIs

CPython is old, the code evolved. Some functions became useless and so should be removed. But backward compatibility matters in Python, so we need a transition period with a deprecation process.

The deprecation can be:

• Emitted at runtime: DeprecationWarning
• At the compilation: Py_DEPRECATED()
• In the documentation

Functions that should deprecated:

• Unicode functions using Py_UNICODE type
• Py_VA_COPY(): use directly standard va_copy(), see email
• Py_MEMCPY(): kept for backwards compatibility

See:

• https://bugs.python.org/issue19569