CI: Avoid GH throttling via Consolidation

#### Issue:
Our CI can fail due to GitHub throwing a 429.
i.e: Too many CI actions tried to pull from the repo and GitHub throttled us.

There are two causes of this:
1. We use GH Workflow Matrices, which spawn a job per entry
2. Every Dependabot PR updates on a merge

Thus the requests to pull the Repo from GitHub is:
```
|dependabot_prs| x 79
```
Where 79 is the current number of individual GitHub actions created through our matrix use.

#### A Solution:
Too prevent this in the future, we should consolidate are GitHub Workflow Jobs.

Technically, a Job could run all the tox envs for a OS Version,
even across Python versions.

That would dramatically reduce the requests to pull the repo down.

This reduce the number GitHub Jobs and increase the Steps per Job.

We would loose some of our high level test reporting,
but I believe that is acceptable.

_Originally posted by @texastony in https://github.com/aws/aws-encryption-sdk-python/issues/564#issuecomment-1441033254_

Start by locating the GitHub Workflow matrix definitions and reviewing how tox environments are distributed across operating-system and Python-version jobs. Compare the current job count with a consolidated arrangement, then verify that CI still runs all tox environments and reports failures acceptably without triggering GitHub throttling.

Written by the indexing model from the issue text.