fix: require python 3.7+ (#410)

* chore(python): drop python 3.6

Source-Link: https://github.com/googleapis/synthtool/commit/4f89b13af10d086458f9b379e56a614f9d6dab7b
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-python:latest@sha256:e7bb19d47c13839fe8c147e50e02e8b6cf5da8edd1af8b82208cd6f66cc2829c

* require python 3.7+ in setup.py

* remove python 3.6 sample configs

* exclude templated README

* remove python 3.6 from noxfile

* remove python 3.6 from remaining files

* remove testing/constraints-3.6.txt

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Anthonios Partheniou <partheniou@google.com>
21 files changed