Issue with PyYAML 6

[Warchant]

Current setup.py from master https://github.com/avirshup/DockerMake/blob/master/setup.py#L19 defines dependency: pyyaml >= 5. Which is now PyYAML 6.

Version 6 made breaking change in load function (added mandatory Loader argument), so this package does not work if you just install pip install dockermake.

Proposal to fix is to freeze versions in setup.py, and in requirements.txt to use .. Then, if you do pip install -r requirements.txt it will go to setup.py and will grab deps from there. Or, simply upgrade to pyyaml 6.