Doesn't handle indexed format strings

The following code which I tried to reduce (which I had no idea was possible):

thing = {'a': 1, 'b': 2}
print('{0[a]} and {0[b]}'.format(thing))

leads to

Traceback (most recent call last):
  File "/home/ethan/venv/bin/pyupgrade", line 10, in <module>
    sys.exit(main())
  File "/home/ethan/venv/lib/python3.7/site-packages/pyupgrade.py", line 1396, in main
    ret |= fix_file(filename, args)
  File "/home/ethan/venv/lib/python3.7/site-packages/pyupgrade.py", line 1372, in fix_file
    contents_text = _fix_fstrings(contents_text)
  File "/home/ethan/venv/lib/python3.7/site-packages/pyupgrade.py", line 1348, in _fix_fstrings
    tokens[i] = token._replace(src=_to_fstring(token.src, node))
  File "/home/ethan/venv/lib/python3.7/site-packages/pyupgrade.py", line 1310, in _to_fstring
    name = ''.join((params[k or str(i)], dot, rest))
KeyError: '0[a]'

This is a very esoteric format string usage, but 🤷‍♂️

[merwok]

Just a note that may be useful:

format uses a mini-language where keys for item access are not quoted, but f-strings contain real, arbitrary expressions and require quotes.

>>> info = {"name": "Alice", "age": 36}
>>> "{0[name]} is {0[age]}".format(info)
'Alice is 36'
>>> f"{info[name]} is {info[age]}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'name' is not defined
>>> f"{info['name']} is {info['age']}"
'Alice is 36'

[asottile]

indeed, a bit of an oversight. oddly enough I thought far enough ahead to support dotted names but not brackets