Use environment markers in setup.py

[ianlini]

Describe the bug Using sys.version_info to modify dependencies is not a recommended way according to sdispater/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.

[ianlini]

I can use poetry to install nimbusml 1.8.0 now, but the PR is not merged and sys.version_info is still used.