In November Victor created the Include/cpython directory and moved a decent amount of public (but not limited) API there.  This included the PyInterpreterState struct.  I'd like to move it into the "internal" headers since it is somewhat coupled to the internal runtime implementation.  The alternative is extra complexity.  I ran into this while working on another issue.

Note that the docs indicate that all of the struct's fields are private (and I am not aware of any macros leaking fields into the stable ABI).  So moving it should not break anything (yeah, right!), and certainly not the stable ABI (Victor's move would have done that already).