Suppress syntax warnings on Python 3.12.

Python 3.12 has a new warning for backslash-character pairs that are not
valid escape sequences at the level of string literals, as opposed to in
some interior syntax such as regular expressions
(https://docs.python.org/3/whatsnew/3.12.html#other-language-changes).
Suppress it by using raw strings.

test/testcrypt.py

@@ -308,8 +308,8 @@ def _lex_testcrypt_header(header):
         # And then match a token
         '({})'.format('|'.join((
             # Punctuation
-            '\(',
-            '\)',
+            r'\(',
+            r'\)',
             ',',
             # Identifier
             '[A-Za-z_][A-Za-z0-9_]*',