Handle escaped braces in f-strings

To use a curly brace in an f-string, you must escape it. For example:

  >>> k = 1
  >>> f'{{{k}'
  '{1'

Saving this as a script and running the 'tokenize' module highlights
something odd around the counting of tokens:

  ❯ python -m tokenize wow.py
  0,0-0,0:            ENCODING       'utf-8'
  1,0-1,1:            NAME           'k'
  1,2-1,3:            OP             '='
  1,4-1,5:            NUMBER         '1'
  1,5-1,6:            NEWLINE        '\n'
  2,0-2,2:            FSTRING_START  "f'"
  2,2-2,3:            FSTRING_MIDDLE '{'     # <-- here...
  2,4-2,5:            OP             '{'     # <-- and here
  2,5-2,6:            NAME           'k'
  2,6-2,7:            OP             '}'
  2,7-2,8:            FSTRING_END    "'"
  2,8-2,9:            NEWLINE        '\n'
  3,0-3,0:            ENDMARKER      ''

The FSTRING_MIDDLE character we have is the escaped/post-parse single
curly brace rather than the raw double curly brace, however, while our
end index of this token accounts for the parsed form, the start index of
the next token does not (put another way, it jumps from 3 -> 4). This
triggers some existing, unrelated code that we need to bypass. Do just
that.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #1948