Fallback pytest to 4.6.x for python2

Author: aahung

requirements/dev.txt

@@ -6,7 +6,8 @@ pylint>=1.7.2,<2.0
 pyyaml>=5.3.1
 
 # Test requirements
-pytest>=6.1.1
+pytest>=6.1.1; python_version >= '3.6'
+pytest~=4.6.11; python_version < '3.6' # pytest dropped python 2 support after 4.6.x
 mock>=3.0.5,<4.0.0  # 4.0.0 drops Python 2 support
 parameterized>=0.7.4