Add Python Raw memory helper functions

See #2528.

WIP: MAINT: avoid numpy internals in {sg}et_array_base

Remove duplicated C compiler option from tests that is already set by the test runner via the cpp11 tag.

See #2509.

Technically these functions were around in some of the Python 3.4
prereleases. So add a more accurate version constraint.

To handle the Python version differences, handle the definitions of
`PyMem_Raw*` functions in `ModuleSetupCode`. Then extern them in
`cpython.mem` without using a version check or a specific header.