Support package_data with empty key

[mxr]

Given this minimal setup.py file (which admittedly needs more things to be filled in to be usable, but works for this example):

from setuptools import setup

setup(package_data={"": ["*.yaml"]})

setup-py-upgrade will produce the following setup.cfg:

[options.package_data]
 =
    *.yaml

However this file cannot be read by configparser or similar tools. For example:

$ pre-commit try-repo https://github.com/asottile/setup-cfg-fmt setup-cfg-fmt --files setup.cfg
===============================================================================
Using config:
===============================================================================
repos:
-   repo: https://github.com/asottile/setup-cfg-fmt
    rev: f1de3ef475160e6871983e3f0a9790b2b74b734e
    hooks:
    -   id: setup-cfg-fmt
===============================================================================
[INFO] Initializing environment for https://github.com/asottile/setup-cfg-fmt.
[INFO] Installing environment for https://github.com/asottile/setup-cfg-fmt.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
setup-cfg-fmt............................................................Failed
hookid: setup-cfg-fmt

Traceback (most recent call last):
  File "/var/folders/91/k4gwvn_n11v22vskgsvpm3fr0000gq/T/tmpdddrjpuh/repozbmcoj8d/py_env-python3/bin/setup-cfg-fmt", line 11, in <module>
    sys.exit(main())
  File "/var/folders/91/k4gwvn_n11v22vskgsvpm3fr0000gq/T/tmpdddrjpuh/repozbmcoj8d/py_env-python3/lib/python3.6/site-packages/setup_cfg_fmt.py", line 341, in main
    max_py_version=args.max_py_version,
  File "/var/folders/91/k4gwvn_n11v22vskgsvpm3fr0000gq/T/tmpdddrjpuh/repozbmcoj8d/py_env-python3/lib/python3.6/site-packages/setup_cfg_fmt.py", line 237, in format_file
    cfg.read_string(contents)
  File "/Users/mxr/.pyenv/versions/3.6.5/lib/python3.6/configparser.py", line 723, in read_string
    self.read_file(sfile, source)
  File "/Users/mxr/.pyenv/versions/3.6.5/lib/python3.6/configparser.py", line 718, in read_file
    self._read(f, source)
  File "/Users/mxr/.pyenv/versions/3.6.5/lib/python3.6/configparser.py", line 1111, in _read
    raise e
configparser.ParsingError: Source contains parsing errors: '<string>'
	[line  2]: ' =\n'
	[line  3]: '    *.yaml\n'

[asottile]

Nice! Looks like the syntax for this is * = *.yaml

https://setuptools.readthedocs.io/en/latest/setuptools.html#configuring-setup-using-setup-cfg-files

[hugovk]

See #61 for a similar one.