[ESI] Add readable `__version__` attribute to esiaccel package

[mortbopet]

Currently, neither __init__.py nor any of the setup/wheel build stuff for the esiaccel python package defines a __version__ attribute. This would be handy for users to be able to runtime verify that a required esiaccel package is available, i.e. through

import esiaccel
print(esiaccel.__version__)

There seems to be a variety of ways/options to inject this, based on how the wheel is built - some inspiration.