Cannot build with Python 3.7+

[pcotret]

There's an issue with _PyImport_FixupBuiltin https://github.com/pulp-platform/pulp-riscv-binutils-gdb/blob/master/gdb/python/python.c#L1744

• Here is the official bug: https://sourceware.org/bugzilla/show_bug.cgi?id=23252
• The patch: https://sourceware.org/legacy-ml/gdb-patches/2018-05/msg00863.html
• Another topic from Python devs: https://bugs.python.org/issue33470

_PyImport_FixupBuiltin prototype has been modified a few years ago: python/cpython@428d223