fix: Use different versions of pytest for python 2 and python3 [(#2558)](GoogleCloudPlatform/python-docs-samples#2558)

* fix: Use different versions of pytest for python 2 and python3

* fix: delete extra pytest dep

* fix: update pytest dependencies in requirements.txt

Author: crwilcox

samples/snippets/requirements.txt

@@ -1,6 +1,7 @@
 google-cloud-pubsub==1.0.0
 google-cloud-containeranalysis==0.3.0
 grafeas==0.1.0
-pytest==5.1.3
+pytest==5.3.0; python_version > "3.0"
+pytest==4.6.6; python_version < "3.0"
 flaky==3.6.1
 mock==3.0.5