PEP 7: Add note about greppable exception messages (#2605)

Co-authored-by: CAM Gerlach <CAM.Gerlach@Gerlach.CAM>

pep-0007.txt

@@ -71,6 +71,12 @@ Code lay-out
   previous rule together don't give you enough room to code, your code
   is too complicated -- consider using subroutines.
 
+* Strive to keep exception message string literals on one line,
+  in order to make them easily greppable.
+  If your exception message breaks the 79 character line width rule,
+  try to mitigate it by reducing indent, or just bend that rule.
+  (Practicality beats purity.)
+
 * No line should end in whitespace.  If you think you need significant
   trailing whitespace, think again -- somebody's editor might delete
   it as a matter of routine.