Rename python pipset `@base_pip3` -> `@envoy_pip3`

[phlax]

I have been using tooling that is setup in envoy more recently in related repos (mobile/website/mobile-website)

I had thought to potentially rename the pipset to just pip3 but on 2nd thoughts this is envoy's pipset so should be named that

[phlax]

posting before making this change as other downstream repos may be affected by this name change

[github-actions]

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