add python3-dev package (#13)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

python3-dev/python3_dev/__init__.py

@@ -0,0 +1,9 @@
+import os
+
+DIR = os.path.join(os.path.dirname(__file__), "install")
+INCLUDE_DIR = os.path.join(DIR, "include")
+
+
+def smoketest():
+  assert os.path.isfile(os.path.join(INCLUDE_DIR, "Python.h")), "Python.h not found"
+  assert os.path.isfile(os.path.join(INCLUDE_DIR, "pyconfig.h")), "pyconfig.h not found"