PyFutureFeatures should not be public

[iritkatriel]

PyFutureFeatures is defined in Include/cpython/compile.h. If it not used in anything which is user-facing, so there is no reason for anyone to use it. It should be private.

Linked PRs

• gh-117411: move PyFutureFeatures to pycore_symtable.h and make it private #117412