Add requirements.txt and .python-version to root

[stefan-pdx]

Hi,

It'd be nice to have the following in the CWL codebase:

• A requirements.txt file at project root to list Python dependencies needed to run test-suite.
• A .python-version to the root to specify the preferred Python version for development. (see pyenv.

[mr-c]

Moved to common-workflow-language/cwltool#5