[Mellanox] Add python3 support for Mellanox platform API

[Junchao-Mellanox]

- Why I did it

python2 is end of life and SONiC is going to support python3. This PR is going to support:

1. Mellanox SONiC platform API python3 support
2. Install both python2 and python3 verson of Mellanox SONiC platform API or pmon and host side

- How I did it

1. Change docker-platform-monitor make files to install both python2 and python3 whl
2. Change Mellanox SONiC platform API code to support both python2 and python3

- How to verify it

Run platform regressions on a few platforms

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[jleveque]

@Junchao-Mellanox: Once this PR is merged, are we good to merge #5886?

[Junchao-Mellanox]

@Junchao-Mellanox: Once this PR is merged, are we good to merge #5886?

@keboliu I suppose you are working on the system health python3 migration, could you answer this comment?

[keboliu]

@Junchao-Mellanox: Once this PR is merged, are we good to merge #5886?

@keboliu I suppose you are working on the system health python3 migration, could you answer this comment?

generally, system-health has a dependency on platform API python3 lib, and when it ready system-health should be able to run, however, I see that system-health itself still has some issue in terms of string handling. left comments in another PR #5886