gdb/python: rework how the disassembler API reads the result object

This commit is a refactor ahead of the next change which will make
disassembler styling available through the Python API.

Unfortunately, in order to make the styling support available, I think
the easiest solution is to make a very small change to the existing
API.

The current API relies on returning a DisassemblerResult object to
represent each disassembled instruction.  Currently GDB allows the
DisassemblerResult class to be sub-classed, which could mean that a
user tries to override the various attributes that exist on the
DisassemblerResult object.

This commit removes this ability, effectively making the
DisassemblerResult class final.

Though this is a change to the existing API, I'm hoping this isn't
going to cause too many issues:

  - The Python disassembler API was only added in the previous release
    of GDB, so I don't expect it to be widely used yet, and

  - It's not clear to me why a user would need to sub-class the
    DisassemblerResult type, I allowed it in the original patch
    because at the time I couldn't see any reason to NOT allow it.

Having prevented sub-classing I can now rework the tail end of the
gdbpy_print_insn function; instead of pulling the results out of the
DisassemblerResult object by calling back into Python, I now cast the
Python object back to its C++ type (disasm_result_object), and access
the fields directly from there.  In later commits I will be reworking
the disasm_result_object type in order to hold information about the
styled disassembler output.

The tests that dealt with sub-classing DisassemblerResult have been
removed, and a new test that confirms that DisassemblerResult can't be
sub-classed has been added.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Tom Tromey <tom@tromey.com>