Unit tests are not run against python3.8 #31

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Environment details

• OS type and version: Arch linux
• Python version: 3.8.2
• pip version: 20.0.2 (installed in virtual env)
• google-cloud-datastore version: master

Steps to reproduce

1. Run unit tests on machine with only python3.8 installed
2. Tests don't run

NOTE: This issue is being created so that it can be attached to a PR.