templater: strictly parse leading backslashes of '{' (issue4569) (BC) Because double backslashes are processed as a string escape sequence, '\\{' should start the template syntax. On the other hand, r'' disables any sort of \-escapes, so r'\{' can go either way, never start the template syntax or always start it. I simply chose the latter, which means r'\{' is the same as '\\{'.