This looks like a serious regression in 3.7.  @ned.deily - I'm marking this as a release blocker, but feel free of course to downgrade it.

Run the following as

$ python3.6 badconfig.py
Hey, it works!

$ python3.7 badconfig.py
Traceback (most recent call last):
  File "../badconfig.py", line 77, in <module>
    fileConfig(inifile.name, defaults=GUNICORN_DEFAULTS)
  File "/Users/barry/projects/python/cpython/Lib/logging/config.py", line 65, in fileConfig
    cp = configparser.ConfigParser(defaults)
  File "/Users/barry/projects/python/cpython/Lib/configparser.py", line 639, in __init__
    self._read_defaults(defaults)
  File "/Users/barry/projects/python/cpython/Lib/configparser.py", line 1212, in _read_defaults
    self.read_dict({self.default_section: defaults})
  File "/Users/barry/projects/python/cpython/Lib/configparser.py", line 754, in read_dict
    self.set(section, key, value)
  File "/Users/barry/projects/python/cpython/Lib/configparser.py", line 1200, in set
    super().set(section, option, value)
  File "/Users/barry/projects/python/cpython/Lib/configparser.py", line 895, in set
    value)
  File "/Users/barry/projects/python/cpython/Lib/configparser.py", line 403, in before_set
    "position %d" % (value, tmp_value.find('%')))
ValueError: invalid interpolation syntax in "{'generic': {'format': '%(asctime)s [%(process)d] [%(levelname)s] %(message)s', 'datefmt': '[%Y-%m-%d %H:%M:%S %z]', 'class': 'logging.Formatter'}}" at position 26

I'm still investigating, but wanted to get the bug filed asap.