[aks-preview]: fix python3-only super() syntax

[mboersma]

Restores the old, Python2-compatible syntax for calling super().

Fixes Azure/AKS#908

cc: @zqingqing1 @xizhamsft

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[azuresdkci]

If this PR is for a new extension or change to an existing extension, use the following to try out the changes in this PR:

docker run -it microsoft/azure-cli:latest
export EXT=<NAME>
pip install --upgrade --target ~/.azure/cliextensions/$EXT "git+https://github.com/mboersma/azure-cli-extensions.git@fix-aks-python2#subdirectory=src/$EXT&egg=$EXT"

[zqingqing1]

@mboersma, seems like it will block customer which is using python 3.7

super(type, obj): obj must be an instance or subtype of type
Traceback (most recent call last):
File "/usr/local/Cellar/azure-cli/2.0.49/libexec/lib/python3.7/site-packages/knack/cli.py", line 197, in invoke
cmd_result = self.invocation.execute(args)
File "/usr/local/Cellar/azure-cli/2.0.49/libexec/lib/python3.7/site-packages/azure/cli/core/commands/init.py", line 262, in execute
self.commands_loader.load_arguments(command)
File "/usr/local/Cellar/azure-cli/2.0.49/libexec/lib/python3.7/site-packages/azure/cli/core/init.py", line 254, in load_arguments
loader.load_arguments(command) # this adds entries to the argument registries
File "/Users/qizhe/cli-extension/azure-cli-extensions/src/aks-preview/azext_aks_preview/init.py", line 34, in load_arguments
super(ContainerServiceCommandsLoader, self).load_arguments(command)
TypeError: super(type, obj): obj must be an instance or subtype of type

[mboersma]

@zqingqing1 in python 3, super() made its two arguments optional, but the old syntax is compatible with Python 2 and Python 3. The previous code wouldn't work in Python2, which az must stay compatible with.

We use the two-argument super() syntax in the methods above and below this one. What's different about this function?

[zqingqing1]

I don't know why, but looks like old syntax is not compatible with python 3.
So I will make change to check the version of python, and then inherit the super() for different version.

This should work.

def load_arguments(self, command):
        from sys import version_info
        if version_info[0] < 3:
            super(ContainerServiceCommandsLoader, self).load_arguments(command)
        else:
            super().load_arguments(command)

        from ._params import load_arguments
        load_arguments(self, command)

[mboersma]

I don't know why either, but I agree that should work. :-)