Over on the python-on-a-chip group, James Snyder posted these links to Cross-Platform, Open Source STM32F serial Flash bootloaders. I have paraphrased two of his posts (I hope this is okay), and reposting here. I hope this is useful.
Actually there are a few cross-platform programs that will flash using
The first is pretty minimal, written in C, I believe it works on Linux, OS X, and maybe Windows.
The second uses pyserial, it's somewhat slow and perhaps not the greatest python code, but it works.
The 3rd is a fairly recent one, it's written in C and probably is more feature complete than the first.
I don't actually know much about sloadhost, but I assume it probably works as well.