requirements: fix pytest version for python 2 vs 3

requirements.txt

@@ -41,7 +41,8 @@ voeventlib==1.2; python_version >= '3'
 # problematic API changes in 2.3.1 and beyond that will need to be handled
 # appropriately.
 pyparsing==2.3.0
-pytest<5
+pytest<5; python_version < '3'
+pytest==5.1.2; python_version >= '3'
 pytest-cov==2.6.1
 pytest-django==3.4.8
 pytz==2018.9