fix: make test just work for different python versions

Specifically cmd tasks may now use the following operations like in bash
- `:-` fallback value, e.g. ${AWS_REGION:-us-east-1}
- `:+` value replacement, e.g. ${AWESOME:+--awesome-mode}

This was done by adding AST parser support for Param Expansion operators

Also:
- Fix bug in param expansion logic for pure whitespace param values
- Add env paramater to PoeThePoet class to override os.environ for testing