python does not release filehandles to logfile

You need to call .close() on the filehandler.

When your Run class completes, call:

handlers = self.log.handlers[:]
for handler in handlers:
    self.log.removeHandler(handler)
    handler.close()

A file handler will automatically re-open the configured filename every time a new log message arrives, so calling handler.close() may sometimes appear futile. Removing the handler from the logger stops future log records from being sent to it; in the above code we do this first, to avoid an untimely log message from another thread reopening the handler.

Another answer here suggest you use logging.shutdown(). However, all that logging.shutdown() does is call handler.flush() and handler.close(), and I'd not recommend using it. It leaves the logging module in a state where you can't use logging.shutdown() again, not reliably.

You can also shutdown the logging completely. In that case, file handles are being released:

logging.shutdown()

It closes opened handles of all configured logging handlers.

I needed it to be able to delete a log file after a unit test is finished and I was able to delete it right after the call to the logging.shutdown() method.

I was using an interactive Python environment (Spyder). Apparently, Spyder uses logging internally. So, logging.shutdown() does not produce the desired effect. The next execution of the same program doubled log records, the 3rd execution tripled them, etc. Handlers are apparently not removed by shutdown() in this environment. Also, I did not disrupt Spyder in any way by issuing an explicit shutdown() call. Puzzling.

Martijn's code to explicitly close and remove the handlers, one at a time, did work in the Spyder environment.