force build-ext build

it seems that what worked in the debug branch is not working in the
main branch, even though the commit IDs are exactly the same. the RTD environment doesn't seem really reliable...

besides, we want to build extensions before the rest, so should run it
first, in order to have msgpack loaded.

setup.py

@@ -220,6 +220,7 @@ class build_py_custom(build_py):
     def run(self):
         super().run()
         self.announce('calling custom build steps', level=log.INFO)
+        self.run_command('build_ext')
         self.run_command('build_api')
         self.run_command('build_usage')