Brittle in which way? I wouldn't expect a maintenance release of an old python version to add support for syntax from a newer version, or a new version removing syntax for current syntax (if that happens we are on a new major version and failing is entirely acceptable)
What if someone makes a change to the pattern because they are maintaining the code but don't understand it? What if a future version of Python stops accepting that as valid syntax?
It's too clever by half.
If python 4 comes along and stops supporting that syntax then it's fairly likely that the rest of the code will be broken too. That doesn't sound like a huge downside to me.