Emit the correct indentation of sequence

[dota17]

Modify the output format of Custom indent in sequence by adding the is_qequence parameter in write_indicator().
Refer to #234, The test case is:

doc = """
%TAG !E! !SSSSS
---
- hosts: localhost
  vars:
    yaml:
      single_attr: value1
      list_of_dict_attr:
        - attr1: value1
          attr2: value2
          attr3:
            - item1
            - item2
  tasks:
    - debug: msg="{{ yaml | to_nice_yaml }}"
- 
  - aaa
  - ccc
...

the result is shown below:

print(yaml.dump(data, indent=6, allow_unicode=True, default_flow_style=False))

    - hosts: localhost
      tasks:
          - debug: msg="{{ yaml | to_nice_yaml }}"
      vars:
            yaml:
                  list_of_dict_attr:
                      - attr1: value1
                        attr2: value2
                        attr3:
                            - item1
                            - item2
                  single_attr: value1
    -     - aaa
          - ccc

print(yaml.dump(data, indent=3, allow_unicode=True, default_flow_style=False))

 - hosts: localhost
   tasks:
    - debug: msg="{{ yaml | to_nice_yaml }}"
   vars:
      yaml:
         list_of_dict_attr:
          - attr1: value1
            attr2: value2
            attr3:
             - item1
             - item2
         single_attr: value1
 -  - aaa
    - ccc

In brief, This output format is more consistent with spec.

Do I need to add the testcases in PR? @perlpunk

[perlpunk]

I guess a test would be nice :)
But I'm not maintaining PyYAML or making any decisions.

[perlpunk]

One thing to keep in mind is that the output of libyaml is different. So there would be a difference between yaml.Dumper and yaml.CDumper.
That will probably be an issue with a test.

[JobaDiniz]

will this ever be merged?