chore(test): add python 3.10 into github actions

[Tomcli]

Which issue is resolved by this Pull Request:
Resolves #

Description of your changes:
KFP is now testing python 3.10 as well since 3.7 is going to be out of support in June. Hence add the 3.10 test here and move away all the default env to 3.8. We will drop 3.7 tests and support in June when Python community officially dropped it.

Environment tested:

• Python Version (use python --version):
• Tekton Version (use tkn version):
• Kubernetes Version (use kubectl version):
• OS (e.g. from /etc/os-release):

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[Tomcli]

/assign @yhwang

[yhwang]

/lgtm

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Tomcli, yhwang

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [Tomcli,yhwang]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment