MNT: Forgot to add set_traceback.pxd

indexed_gzip/set_traceback.pxd

@@ -0,0 +1,10 @@
+#
+# Cython declaration for the PyException_SetTraceback function.
+# This function is in the Python C API, but is not in the built-in
+# Cython declarations.
+#
+
+from cpython.ref cimport PyObject
+
+cdef extern from "Python.h":
+    PyObject* PyException_SetTraceback(PyObject* ex, PyObject* tb)