build(python): remove python 3.7 from kokoro Dockerfile

[parthea]

A lot of python repositories have a failing Kokoro docs-presubmit check with error gpg: keyserver receive failed: No name. For example, googleapis/python-asset#227

The issue is that the fetch_gpg_keys.sh script is failing in the Docker file here because the key here is no longer available on the keyserver.

This PR removes fetch_gpg_keys.sh as, in most repositories, we don't need to install additional python versions. We can use the default python version in the ubuntu 20.04 base image which is python 3.8.10.

[plamut]

Looks good, thanks! And just in the right time, I just notice the failing docs-presubmit tests myself, too.