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

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

* fix: delete extra pytest dep

* fix: update pytest dependencies in requirements.txt

googleapis/python-containeranalysis,googleapis/python-containeranalysis