Run mocked Gremlin tests in CI

[j6k4m8]

Summary

• add a gremlin optional dependency for gremlinpython
• install the Gremlin extra in the Python CI matrix so mocked backend tests execute
• update the dependency lockfile

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks

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_{Comparing ci/gremlin-extra-86 (4e81cee) with master (42049d8)}