Add requirements.txt and .python-version to root

[mr-c]

From common-workflow-language/common-workflow-language#112 & common-workflow-language/common-workflow-language#113

[manu-chroma]

requirements.txt is in the root and versioning in setup.py.

do you want a separate .python-version file, we can also have __version__ variable in cwltool/__init__.py

multiple ways to do this. what's your take on this?

[manu-chroma]

@mr-c this seems trivial. I want to know your take on this one

[mr-c]

@manu-chroma go for it. for .python-version see https://github.com/pyenv/pyenv#choosing-the-python-version
I don't think this issue is about a __version__ variable in cwltool/__init__.py

[manu-chroma]

There is very little documentation for .python-version online but from what I understand this is used to indicate what specific Python runtime we'd like to use.

I tried searching for this file on GitHub repos and it seems this is something which should be local config of the user and not present upstream. https://github.com/search?q=.python-version&type=Code&utf8=%E2%9C%93

@mr-c what do you think?