title: Why is the interpolation key 'asctime' not a valid option name?
body: <p>I have a <code>setup.cfg</code> with</p>
  <pre class="lang-ini prettyprint-override"><code>[tool:pytest]
  doctest_encoding = utf-8
  log_cli = 0
  log_cli_level = INFO
  log_cli_format = %(asctime)s [%(levelname)8s] %(message)s (%(filename)s:%(lineno)s)
  log_cli_date_format=%Y-%m-%d %H:%M:%S
  </code></pre>
  <p>and I get the following error message</p>
  <pre><code>$ pip install .[all]
  Processing /some/repo
      ERROR: Complete output from command python setup.py egg_info:
      ERROR: Traceback (most recent call last):
        File &quot;&lt;string&gt;&quot;, line 1, in &lt;module&gt;
        File &quot;/tmp/pip-req-build-599014dc/setup.py&quot;, line 72, in &lt;module&gt;
          setup(**config)
        File &quot;/usr/local/lib/python3.7/site-packages/setuptools/__init__.py&quot;, line 144, in setup
          _install_setup_requires(attrs)
        File &quot;/usr/local/lib/python3.7/site-packages/setuptools/__init__.py&quot;, line 137, in _install_setup_requires
          dist.parse_config_files(ignore_option_errors=True)
        File &quot;/usr/local/lib/python3.7/site-packages/setuptools/dist.py&quot;, line 695, in parse_config_files
          self._parse_config_files(filenames=filenames)
        File &quot;/usr/local/lib/python3.7/site-packages/setuptools/dist.py&quot;, line 599, in _parse_config_files
          val = self._try_str(parser.get(section, opt))
        File &quot;/usr/local/lib/python3.7/configparser.py&quot;, line 799, in get
          d)
        File &quot;/usr/local/lib/python3.7/configparser.py&quot;, line 394, in before_get
          self._interpolate_some(parser, option, L, value, section, defaults, 1)
        File &quot;/usr/local/lib/python3.7/configparser.py&quot;, line 434, in _interpolate_some
          option, section, rawval, var) from None
      configparser.InterpolationMissingOptionError: Bad value substitution: option 'log_cli_format' in section 'tool:pytest' contains an interpolation key 'asctime' which is not a valid option name. Raw value: '%(asctime)s [%(levelname)8s] %(message)s (%(filename)s:%(lineno)s)'
      ----------------------------------------
  ERROR: Command &quot;python setup.py egg_info&quot; failed with error code 1 in /tmp/pip-req-build-599014dc/
  ERROR: Job failed: exit code 1
  </code></pre>
  <p>when it is executed in a Docker image based on <code>python:3.7-slim</code> on Gitlab.</p>
  <p>When I remove the two lines</p>
  <pre><code>log_cli_format = %(asctime)s [%(levelname)8s] %(message)s (%(filename)s:%(lineno)s)
  log_cli_date_format=%Y-%m-%d %H:%M:%S
  </code></pre>
  <p>it works also in the Docker container running on Gitlab. It works for both on my machine (Ubuntu 18.04)</p>
  <p>Why does that happen and how do I fix it?</p>