Update Python templates to use RemoteBackend construct

[ansgarm]

Currently both Python templates use an override to specify a remote backend for Terraform state:

stack.add_override('terraform.backend', {
  'remote': {
    'hostname': 'app.terraform.io',
    'organization': '${organizationName}',
    'workspaces': {
      'name': '${workspaceName}'
    }
  }
})`);

Whereas the other templates (e.g. typescript) already use a dedicated construct for this:

new RemoteBackend(stack, {
  hostname: 'app.terraform.io',
  organization: '${organizationName}',
  workspaces: {
    name: '${workspaceName}'
  }
});`);

We should update the Python templates to use the construct instead of an override.

[ansgarm]

Closed via #669

[github-actions]

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