build(python): remove python 3.7 from kokoro Dockerfile (#803)

Source-Link: https://github.com/googleapis/synthtool/commit/e44dc0c742b1230887a73552357e0c18dcc30b92
Post-Processor: gcr.io/repo-automation-bots/owlbot-python:latest@sha256:5ff7446edeaede81c3ed58b23a4e76a5403fba1350ce28478045657303b6479d
3 files changed