Update PyTest to v5.x to avoid attr incompatibility. (#2428)
Update PyTest to v5.x to avoid attr incompatibility. (#2428)

* recent 19.2.0 attr release is incompatible with our version of pytest

* update pytest to v5

GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples,GoogleCloudPlatform/python-docs-samples