various: mark files with the usual license tags

For .clang-format and pytest.ini, MIT-0 is used because people might want
to copy those.

test/test-dlopen-note.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
 """
 Compare statically linked dlopen symbols against shared library ELF notes.