Deprecate @test_helper decorator

[richardm-stripe]

The @test_helper decorator is somewhat awkward and will make it difficult to add type annotations.

This PR deprecates the decorator and

1. adds a getter

  @property
  def test_helpers(self):
      return self.TestHelpers(self)

2. sets ._resource_cls directly

MyResource.TestHelper._resource_cls = MyResource

and
3. Puts def _static_request and def _static_request_stream in the APIResourceTestHelpers parent class instead of being added dynamically by the decorator.

[Silvrbckw]

Reviewed and Commit