Not all "can't" in error messages were replaced by "cannot".

$ find Parser Python Objects Modules -name '*.[ch]' | xargs egrep '".*[Cc]an'\''t.*"' | wc -l
181
$ find Parser Python Objects Modules -name '*.[ch]' | xargs egrep '".*[Cc]annot.*"' | wc -l
247

$ find Lib -name '*.py' | xargs egrep '(["'\'']).*[Cc]an'\''t.*\1' | wc -l
239
$ find Lib -name '*.py' | xargs egrep '(["'\'']).*[Cc]annot.*\1' | wc -l
602

$ find Doc -name '*.rst' | xargs egrep '[Cc]an'\''t' | wc -l
180
$ find Doc -name '*.rst' | xargs egrep '[Cc]annot' | wc -l
482