gdb/python: remove Py_TPFLAGS_BASETYPE from gdb.UnwindInfo

gdb/python: remove Py_TPFLAGS_BASETYPE from gdb.UnwindInfo

It is not currently possible to directly create gdb.UnwindInfo
instances, they need to be created by calling
gdb.PendingFrame.create_unwind_info so that the newly created
UnwindInfo can be linked to the pending frame.

As such there's no tp_init method defined for UnwindInfo.

A consequence of all this is that it doesn't really make sense to
allow sub-classing of gdb.UnwindInfo.  Any sub-class can't call the
parents __init__ method to correctly link up the PendingFrame
object (there is no parent __init__ method).  And any instances that
sub-classes UnwindInfo but doesn't call the parent __init__ is going
to be invalid for use in GDB.

This commit removes the Py_TPFLAGS_BASETYPE flag from the UnwindInfo
class, which prevents the class being sub-classed.  Then I've added a
test to check that this is indeed prevented.

Any functional user code will not have any issues with this change.

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