trappy: set the version using an explicit file

There is no pythonic way of specifying the version of a project, [0] describes 7 (seven!) ways of doing it. We were currently using method 5, setting the value in setup.py and using pkg_resources to get it from the installed version. This works ok if you have installed the package using "python setup.py" or pip, but fails if you are importing trappy from a checkout, which is what lisa do. Even worse, if you import it from lisa but have an old trappy version installed, trappy.version will tell you the version of the installed trappy, not the one you have imported and are using.

Switch to use a version.py file that's distributed with the project (method 3). trappy.version now reads the imported trappy's version, which is what you want. setup.py and the documentation read the file when they are installing so as to avoid repeating the version number there.

[0] https://packaging.python.org/en/latest/single_source_version/