Add libraries to the python-multi image.

[chalmerlowe]

I often use python-multi to perform troubleshooting on various googleapis libraries.
As part of that troubleshooting, I routinely use the following tools to assist with making edits, running tests (especially nox.sessions), etc.

As such, I typically start my sessions by:

• installing nox
• installing vi

Opening this for discussion.
I would like to add both nox and vi to the base build for python-multi.

Will issue a PR related to this shortly.

UPDATE:

In addition, in order to facilitate the install of conda/mamba-based virtual environments, I added conda/mamba to the python-multi Image.

Per request, I added the use of hashed requirements.in/txt files to ensure that exactly the right versions of dependencies were being use.

However... as I began editing the four Dockerfiles for Python-based Docker Images, I noticed that there were several other potential issues/discrepancies that I figured I would ask about:

What is the design principle behind choosing the default version of Python for the various Docker Images:
One has Python 3.6, one has Python 3.8, etc.