See my comment from over a year ago: <https://bugs.python.org/issue15873#msg273609>.  The proposed patch did not have a C implementation, but we can use the same approach as with strptime and call Python code from C.  If users will start complaining about performance, we can speed it up in later releases.  Also the new method needs to be documented.  Overall, it does not seem to require more than an hour of work from a motivated developer, but the people who contributed to the issue in the past seem to have lost their interest.