I have the Python modules a.py and b.py in the same directory.
How can I reliably import b.py from a.py, given that a.py may have been imported from another directory or executed directly? This module will be distributed so I can't hardcode a single path.

I've been playing around with `__file__`, sys.path and os.chdir, but it feels messy. And `__file__` is not always available.