Unclear if this regression (from this patch) is intentional or not:

```
$ python3.6 -c 'import configparser; configparser.ConfigParser(defaults={"a": None})'
$ python3.7 -c 'import configparser; configparser.ConfigParser(defaults={"a": None})'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/configparser.py", line 638, in __init__
    self._read_defaults(defaults)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/configparser.py", line 1216, in _read_defaults
    self.read_dict({self.default_section: defaults})
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/configparser.py", line 753, in read_dict
    self.set(section, key, value)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/configparser.py", line 1197, in set
    self._validate_value_types(option=option, value=value)
  File "/usr/local/Cellar/python/3.7.0/Frameworks/Python.framework/Versions/3.7/lib/python3.7/configparser.py", line 1182, in _validate_value_types
    raise TypeError("option values must be strings")
TypeError: option values must be strings
```