Improve extraction of version

[mathisloevenich]

It would be nice to get the current version with:
htcanalyze --version dynimically with by the latest tag number.

The current solution uses pip, under the condition that the script was installed with pip:

def version():
    """Get version from setup.py."""
    output = subprocess.check_output(
        "pip show htcanalyze | grep Version",
        shell=True
    )
    return output.decode('utf-8').split()[-1]

There are some modules that are able to do so, but they seem to add more complexety than they solve.
Look at the column: Write discovered version to a file of the comparisons described here.

I don't like the standard way of defininig a __version__ variable as this is not bounded to the lastest tag version