Downgrade pytest version for Python2 compatibility

Author: hackermd

requirements_test.txt

@@ -1,3 +1,3 @@
-pytest==5.0.1
+pytest==4.6.5
 pytest-flake8==1.0.4
 pytest-localserver==0.5.0

setup.py

@@ -53,7 +53,7 @@
         ]
     },
     tests_require=[
-        'pytest>=5.0',
+        'pytest>=4.6',
         'pytest-localserver>=0.5',
         'pytest-flake8>=1.0',
         'tox>=3.13',