Method ```configparser.RawConfigParser.set()``` has optional parameter *value* with default value ```None``` resulting in the behavior that actually trying to set a config parameter to ```None``` will not be propagated to ```Interpolation.before_set()```. In fact, since it uses ```if value:``` and not ```if value is None:``` none of the non-truthy values will be passed through.

Suggested commit [8e008be](https://github.com/HStry/cpython/commit/8e008bea0cf6bd3c698b333fd39a383e124fe026) using already established ```_UNSET``` singleton, but that appears to break compatibility elsewhere.