add convenient entry point for cwltool

[mr-c]

@tetron I would suggest using this (cwltool.py) instead of cwltool/main.py to solve the issues Curoverse was having since the move to relative imports.

As a reminder, here is the other workaround: setup.py install or setup.py develop and use the cwltool that gets install to the appropriate bin/ directory.

[prismofeverything]

What was the motivation for relative imports? It breaks being able to run anything unless you are running inside the package. What is the benefit side of this tradeoff?

[mr-c]

Hello @prismofeverything,

Relative imports are required for Python 3 compatibility, and the use of any Python 3 based tools, such as the MyPy static type checker

This PR fixes the ability to run cwltool without otherwise installing or turning on develop mode. I'm going to go ahead and merge this as I see no downsides.

Is there something I missed?

[prismofeverything]

Ah, thanks for explaining! I forgot python 3 even existed. Yes, with your addition things run again, thanks : )

[mr-c]

Glad to hear everything is working :-) Thank you for reminding me that I needed to merge this.