{Packaging} Keep py3 model files

[jiasli]

An alternative solution to #21143

Symptom

When building Windows MSI, _models_py3.py files are removed to reduce package size.

• _models.py are both Python 2 and 3 compatible
• _models_py3.py are only Python 3 compatible

Since 2022-01, Python 2 support has been dropped by Python SDK (Azure/azure-sdk-for-python#20691).

• _models.py files are removed from new version Python SDKs
• That is, only _models_py3.py files are kept in Python SDKs

So far, this change has been done on azure-mgmt-netapp and azure-mgmt-trafficmanager.

However, because CLI's build pipeline removes _models_py3.py files, no model files will exist in the MSI package. This leads to CI failure:

https://dev.azure.com/azure-sdk/public/_build/results?buildId=1332093&view=logs&j=fec5ba3f-cada-52e0-3c40-eb322e7bf2a6&t=50bad18b-08a9-5298-0f82-721960fdcd1c

2022-01-28T05:44:45.9762940Z ERROR: Error loading command module 'network': No module named 'azure.mgmt.trafficmanager.models._models_py3'
...
2022-01-28T05:45:03.8574497Z   File "D:\a\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/mgmt/netapp/__init__.py", line 9, in <module>
2022-01-28T05:45:03.8575668Z   File "D:\a\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/mgmt/netapp/_net_app_management_client.py", line 16, in <module>
2022-01-28T05:45:03.8576594Z   File "D:\a\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/mgmt/netapp/models/__init__.py", line 9, in <module>
2022-01-28T05:45:03.8577404Z ModuleNotFoundError: No module named 'azure.mgmt.netapp.models._models_py3'

Full log: 557.txt

Test guide

1. Remove the current installed Azure CLI from control panel to make the system clean
2. Install the built artifact MSI from build pipeline
3. Check files under C:\Program Files (x86)\Microsoft SDKs\Azure\CLI2\Lib\site-packages\azure\mgmt\trafficmanager\models to make sure _models_py3.pyc exists
4. Run az self-test

Pros and Cons

• The MSI only increases from 48MB to 52MB which is acceptable. With the removal of _models.py files from SDKs in the future, the MSI size will decrease gradually
• If SDK renames _models_py3.py to _models.py in the future, no action will be required on CLI side

TODO

The CI job TestWindowsMSI fails to detect this issue when there is no version update, possibly due to old _models_py3.py files not being removed when installing MSI with the same version. This requires further investigation.