Describe the bug
Using sys.version_info to modify dependencies is not a recommended way according to
python-poetry/poetry#758. We should use environment markers instead. If we use sys.version_info to modify dependencies, some package managers such as Poetry will fail to get the correct dependencies.

To Reproduce
Steps to reproduce the behavior:

1. install poetry
2. in a Python 3.7.4 environment, poetry add nimbusml
3. poetry will try to install enum (0.4.7), which is a dependency for Python 2
4. the installation will fail (I think enum only support Python 2)

Expected behavior
The installation succeed without installing those dependencies for Python 2.

Additional context
The Python package Ray also had the same issue. We can refer to their PR ray-project/ray#3764.