add SPDX-License-Identifier and DCO information

this includes my personal signoff on the MAINTAINERS.md for DCO purposes

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

incorporealcms/mdx/pydot.py

@@ -1,4 +1,7 @@
-"""Serve dot diagrams inline."""
+"""Serve dot diagrams inline.
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+"""
 import base64
 import logging
 import re