chore: use Python 3.8 for docs / docfx sessions (#125)
chore: use Python 3.8 for docs / docfx sessions (#125)

Works around recent changes to the Kokoro 'docs-presubmt' configuration.

See: https://github.com/googleapis/python-api-core/pull/224

Also, pin sphinx version (`docs` / `docfx`) and use 'gcp-sphinx-docfx-yaml' (`docfx`).
googleapis/python-cloud-core,googleapis/python-cloud-core