Now that the string-sig has settled on r"obin" strings, restrict the <letter><string> notation to 'r' and 'R'.
Now that the string-sig has settled on r"obin" strings, restrict the
<letter><string> notation to 'r' and 'R'.

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator