Split pytest requirements based on Python version installed

Author: mdmintz

requirements.txt

@@ -10,7 +10,8 @@ urllib3==1.25.3
 requests>=2.22.0
 selenium==3.141.0
 pluggy>=0.12.0
-pytest>=4.6.4
+pytest>=4.6.4;python_version<"3"
+pytest>=5.0.1;python_version>="3"
 pytest-cov>=2.7.1
 pytest-forked>=1.0.2
 pytest-html>=1.21.1