Ensure that versions are PEP 440 compliant #42
Description
In the long run all of our tooling will need to generate and consume PEP 440 compliant versions. Currently conda supports PEP 440 along with various extensions to the syntax for historical reasons (and in some cases, convenience), but that will eventually become untenable as wheel builds and pip packaging become more central to our workflows. Once dfg becomes a true single source of truth for versions, it would also make sense to add version linting as a validation step to ensure that all versions in dependencies.yaml are compliant. Version normalization is fairly easy to do by simply constructing a packaging.version.Version and ensuring that the string representations have not changed (there may also be a helper function to check this directly).