Make sure tests pass on Windows, add windows to GitHub Actions workflow

[andyk]

Currently, the test_evolutionary_agent test uses *nix specific syntax in the virtualenv.run() command, which is thus failing:

>       virtualenv.run(f"cd {os.getcwd()}/example_agents/evolutionary_agent; "
                       "ls -al; "
                       "pip install -r requirements.txt; "
                       "agentos run --max-iters 5 agent.py "
                       "gym.envs.classic_control.CartPoleEnv")

These commands should use os.sep, os.pardir, etc. to be OS agnostic.