[Python] Codegen for definition with no properties leads to missing models

Similar to #4804

Models with no property are valid according to swagger 2.0 spec:

...

Unlike previous versions of Swagger, Schema definitions can be used to describe primitive and arrays as well.

{
    "type": "string",
    "format": "email"
}

...

but code generator does not support them. My suggestion is to create classes that extends primitive types in this case. e.g., for something like this:

"MyString": {
"Description": "My string description",
"type": "string",
}

generator should generate something similar to:

...
class MyString(string):
   """My string description"""
   pass
...

Also if there is no type, we should assume it is a free range schema and class should extend "object". An example for something like this is v1.Patch in kubernetes in this issue

No source file or test is named. Start by reproducing Python output for Swagger 2.0 definitions with no properties, then trace the definition-handling path and generated-model template; done means primitive and typeless definitions produce valid models rather than being omitted.