Add pmon daemons python3 build support

[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. Build pmon daemons with python3
2. Install and run python3 version pmon daemons

- How I did it

1. Change pmon daemons make files to build bothe python2 and python3 whl
2. Change docker-platform-monitor make files to install both python2 and python3 whl
3. Change pmon docker startup files to start pmon daemons according to the supported platform API version

- How to verify it

1. Manual test to make sure all daemons are up and no error in syslog
2. 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]

I believe there is no need for this python3_daemons variable. I think it is enough to check if API_VERSION == 3. What do we gain from python3_daemons? I feel like it is redundant and it will result in extra cleanup once we completely remove Python 2.

[Junchao-Mellanox]

I understand your concern. In Mellanox platform, some sfp related API depend on a python lib provided by SDK. That lib only supports python2 now (There is plan to migrate it to python3). In that case, xcvrd needs to be run as python2 for now on Mellanox platform. That's the reason I added python3_daemons here.

[jleveque]

I see. I would like to prevent the need for all vendors to temporarily add this info, as Python 3 will be the default moving forward. Maybe it would be better to name the variable python2_daemons and invert the check here (e.g., and 'xcvrd' not in python2_daemons)? This way it is less work to switch to Python 3?

[jleveque]

This PR is opened against your personal fork. Is there a reason, or was this an accident?

[Junchao-Mellanox]

Hi @jleveque , I opened it in my personal repo due to the build issue we discussed before. As we talked, there is a unit test issue on xcvrd when build python3 version wheel. I plan to fix it by skipping the ImportError.

The work around could be change the following code in y_cable.py from:

try:
    import struct

    import sonic_platform.platform
    from sonic_py_common import logger

except ImportError as e:
    raise ImportError(str(e) + " - required module not found")

to

try:
    import struct

    import sonic_platform.platform
    from sonic_py_common import logger

except ImportError as e:
    pass

[Junchao-Mellanox]

sonic-net#6176