gdb: cast all Python extension objects passed to gdbpy_ref_policy to PyObject*

When enabling the Python limited API, pointers to Python C extension
objects can no longer be implicitly converted to 'PyObject *' by the
compiler.

gdbpy_ref_policy is a templated class that provides a generic interface
for incrementing and decrementing the reference counter on the given
object. It is used as a specialisation of the policy parameter in
gdb::ref_ptr, together with PyObject as the parameter type. As a result,
gdbpy_ref_policy always expects an argument derived from PyObject.

This patch fixes the resulting compilation issue by adding an explicit
static_cast to 'PyObject *' before passing the value to Py_INCREF and
Py_DECREF. As a side effect, these casts enforce, at compile time, that
the template type passed to gdbpy_ref_policy is a subclass of PyObject.
To provide a clearer diagnostic when an incorrect type is used, a
static_assert is added to gdbpy_ref_policy, avoiding obscure errors
originating from the static_cast. Finally, all C Python extension types
passed to gdbpy_ref_policy are updated to inherit from PyObject.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23830
Approved-By: Tom Tromey <tom@tromey.com>