python: bump PyCXX to 7.1.0

Bump the embedded copy of PyCXX to 7.1.0, using only the Python2 sources
with a layout that matches the upstream sources (and thus it is easier
to build the sources with no changes).

Adapt krosspython to the new PyCXX:
- adjust the paths to sources, and to includes to the new layout of the
  files
- build the new cxx_exceptions.cxx
- build with -DPYCXX_PYTHON_2TO3 to use all the compatibility functions
  not available in Python 3
- build the C sources as C99, as they use C99 features
- explicitly use PyCxx_ssize_t as type for a couple of methods, instead
  of 'int' used by old versions
- unambiguate one Py::MethodDefExt constructor call