Fix SyntaxWarning in rezconfig.py

The \ in the string literal was generating the following warning every
time the file was parsed:
SyntaxWarning: invalid escape sequence '\ '

This commit uses a raw string literal to avoid interpretation of
backslashes.

src/rez/rezconfig.py

@@ -2,7 +2,7 @@
 # Copyright Contributors to the Rez Project
 
 
-"""
+r"""
 Rez configuration settings. Do not change this file.
 
 Settings are determined in the following way (higher number means higher