setup.py: add parent to sys.path

When using pyproject.toml the parent of setup.py
is not on sys.path by default.

See <https://github.com/pypa/setuptools/discussions/3134>

setup.py

@@ -19,6 +19,7 @@ try:
 except ImportError:
     cythonize = None
 
+sys.path += [os.path.dirname(__file__)]
 import setup_checksums
 import setup_compress
 import setup_crypto