Split pytest-mock requirement by python version

Author: rbpatt2019

requires-testing.txt

@@ -1,7 +1,8 @@
 pytest==6.0.1;python_version>="3.0"
 pytest==4.6.9;python_version=="2.7"
 pytest-sugar==0.9.4
-pytest-mock==3.2.0
+pytest-mock==3.2.0;python_version>="3.0"
+pytest-mock==2.0.0;python_version=="2.7"
 lxml==4.5.0
 selenium==3.141.0
 percy==2.0.2