Both pieces of code are broken. "except BaseException" is wrong because it'll catch KeyboardInterrupt and such, after the file object exists. The mock is wrong because it's introducing an exception that the original code can never raise and was written to assume wouldn't be raised, thus turning a corner case rare bug (fd reuse when a user ^Cs a program) into a bigger problem.
The tempfile code should change to only catch Exceptions, not BaseExceptions; it may opt to additionally catch all BaseExceptions and only unlink the name, but that's probably not sufficient anyway since a KeyboardInterrupt may be raised before the whole try block. Really, the unlink should happen in a wider scope try block, and the _os.close should be more tightly constrained to the _io.open call. And the exception matching should be narrower, because it's safer to leak an fd than to accidentally close it twice.
Then either the test harness should find some other way to implement that, or they can ask the tempfile people to move the _TemporaryFileWrapper out of the try block so they can keep using that hook point, if it hasn't been already by that point.
If there's a strong enough case that this is broken (and the KeyboardInterrupt example is a good one), I think we should file a bug about it.
EDIT: Created issue39318: https://bugs.python.org/issue39318. I think the possibility of this failing due to a KeyboardInterrupt, even if remote, is sufficient to consider this a bug (and with KeyboardInterrupt, it would be extremely subtle and hard to reproduce - more reason to get it fixed).
Looks like his fix didn’t have this bug.
It may have been modified later, his patch was from 2014.