[Python Schema] Support setting namespace for python schema

[gaoran10]

Motivation

Currently, the python schema didn't support setting namespace in the schema definition.

This PR is used to support setting namespace in the python schema definition.

I have two ways to set the namespace for Record, one is to add a new class method for Record to set the namespace, one is to add a new field _avro_namespace for class Record. I select the second way.

for example, add a special field _avro_namespace for the custom Record to add the namespace.

class User(Record):
    _avro_namespace = 'xxx.xxx.xxx'
    name = String()
    age = Integer()

The schema definition is like this.

{
  'name': 'User', 'namespace': 'xxx.xxx.xxx', 'type': 'record', 'fields': [
    {'name': 'age', 'type': ['null', 'int']}, 
    {'name': 'name', 'type': ['null', 'string']}
  ]
}

Modifications

Modify the method schema_info of the class Record to add the namespace field when generating the schema definition.

Verifying this change

Add test to verify the schema definition.

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)
• The rest endpoints: (no)
• The admin cli options: (no)
• Anything that affects deployment: (no)

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[merlimat]

Instead of adding and then deleting, we could just add it only if it's not None

[gaoran10]

Ok

[merlimat]

I was wondering given that this is very specific to Avro, if we should name it as _avro_namespace to make it clear.

[merlimat]

One other thing is that we should add this to the Python schema docs

[gaoran10]

Good idea, I'll the name _avro_namespace.