gdb/python: allow ref_ptr<T, Policy>::new_reference to accept subclasses of T

When ref_ptr<T,Policy>::new_reference() is specialized for 'PyObject'
(i.e. gdbpy_ref<>), it currently requires the argument type to be exactly
'PyObject *'. As a result, pointers to subclasses of 'PyObject' must be
explicitly cast before being passed, making call sites unnecessarily
verbose.

This patch makes ref_ptr<T,Policy>::new_reference() a template method
that accepts both T and subclasses of T, performing the cast to 'T *'
internally when needed. This removes redundant casts at call sites
without changing behavior.

Approved-By: Tom Tromey <tom@tromey.com>