Fix syntax warnings for Python 3.12
https://github.com/Cantera/cantera/commit/25b52d3d1d974d8279026332340d690e42c111c5
--- site_scons/site_tools/subst.py
+++ site_scons/site_tools/subst.py
@@ -130,7 +130,7 @@
 # Replace @key@ with the value of that key, and @@ with a single @
 ##############################################################################
 
-_SubstFile_pattern = "@(?P<key>\w*?)@"
+_SubstFile_pattern = r"@(?P<key>\w*?)@"
 def _SubstFile_replace(env, mo):
     key = mo.group("key")
     if not key:
@@ -167,7 +167,7 @@
 # other defines that you do not desire to be replaced.
 ##############################################################################
 
-_SubstHeader_pattern = "(?m)^(?P<space>\\s*?)(?P<type>#define|#undef)\\s+?@(?P<key>\w+?)@(?P<ending>.*?)$"
+_SubstHeader_pattern = r"(?m)^(?P<space>\\s*?)(?P<type>#define|#undef)\\s+?@(?P<key>\w+?)@(?P<ending>.*?)$"
 def _SubstHeader_replace(env, mo):
     space = mo.group("space")
     type = mo.group("type")