Request the Python stable API when building

We have Python 3.6 as a minimum version. If we set Py_LIMITED_API
to 0x03060000, we'll get the stable python API associated with
versions >= 3.6. This lets users compile once and have the libvirt
binary module be loadable by any Python version >= 3.6, as described
in:

  https://docs.python.org/3/c-api/stable.html

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

setup.py

@@ -87,6 +87,8 @@ def get_module_lists():
     ldflags = get_pkgconfig_data(["--libs-only-L"], "libvirt", False).split()
     cflags = get_pkgconfig_data(["--cflags"], "libvirt", False).split()
 
+    cflags += ["-Wp,-DPy_LIMITED_API=0x03060000"]
+
     module = Extension("libvirtmod",
                        sources=[
                             "libvirt-override.c",