doc/bst2html.py: Fix deprecation warning

This fixes the following:
"
/home/jjardon/buildstream/buildstream/doc/bst2html.py:32: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
  from collections import Mapping
"

doc/bst2html.py

@@ -29,7 +29,7 @@ import sys
 import re
 import shlex
 import subprocess
-from collections import Mapping
+from collections.abc import Mapping
 from contextlib import contextmanager
 from tempfile import TemporaryDirectory