Support cmdclass in setup.cfg

[makeroo]

Having cmdclass defined in setup.cfg causes sdist, or any other, command to fail because its value is not properly parsed into a dict.

How to replicate

setup.cfg example:

[metadata]
name = cmdclass example

[options]
packages = find:
cmdclass =
    sdist = build.CustomSdistCommand

setup.py:

import setuptools
setuptools.setup()

build.py

from setuptools.command.sdist import sdist

class CustomSdistCommand (sdist):
    def run(self):
        print('custom steps...')
        super(sdist, self).run()

error:

❯ python setup.py sdist
Traceback (most recent call last):
  File ".../setup.py", line 5, in <module>
    setuptools.setup()
  File ".../.venv/lib/python3.9/site-packages/setuptools/__init__.py", line 153, in setup
    return distutils.core.setup(**attrs)
  File ".../lib/python3.9/distutils/core.py", line 134, in setup
    ok = dist.parse_command_line()
  File ".../lib/python3.9/distutils/dist.py", line 483, in parse_command_line
    args = self._parse_command_opts(parser, args)
  File ".../.venv/lib/python3.9/site-packages/setuptools/dist.py", line 916, in _parse_command_opts
    nargs = _Distribution._parse_command_opts(self, parser, args)
  File ".../lib/python3.9/distutils/dist.py", line 539, in _parse_command_opts
    cmd_class = self.get_command_class(command)
  File ".../.venv/lib/python3.9/site-packages/setuptools/dist.py", line 750, in get_command_class
    return self.cmdclass[command]
TypeError: string indices must be integers

It happens because self.cmdclass evaluates to the string "sdist = build.CustomSdistCommand" but it should be { "sdist": <class 'build.CustomSdistCommand'> }

Environment details:

❯ python
Python 3.9.0 (v3.9.0:9cf6752276, Oct  5 2020, 11:29:23) 
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

PIP: 21.0.1
setuptools: 53.0.0

Possible solutions:
either change setup.cfg structure introducing options.cmdclass section or fix setuptools/dist.py Distribution._parse_config_files method so that it parses cmdclass value.

Full minimal example:
example.zip