Fix DeprecationWarning about 'invalid escape sequence' in tools/docs Python scripts (#2281)

diff --git a/docs/conf.py b/docs/conf.py
index 585987e..663a816 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -238,7 +238,7 @@
 #'pointsize': '10pt',
 
 # Additional stuff for the LaTeX preamble.
-'preamble': '\DeclareUnicodeCharacter{00A0}{}',
+'preamble': r'\DeclareUnicodeCharacter{00A0}{}',
 
 # Latex figure (float) alignment
 #'figure_align': 'htbp',