fix: revert to using is mapping in Jinja2

[dafyddj]

PR progress checklist (to be filled in by reviewers)

• Changes to documentation are appropriate (or tick if not required)
• Changes to tests are appropriate (or tick if not required)
• Reviews completed

---

What type of PR is this?

Primary type

• [build] Changes related to the build system
• [chore] Changes to the build process or auxiliary tools and libraries such as documentation generation
• [ci] Changes to the continuous integration configuration
• [feat] A new feature
• [fix] A bug fix
• [perf] A code change that improves performance
• [refactor] A code change that neither fixes a bug nor adds a feature
• [revert] A change used to revert a previous commit
• [style] Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc.)

Secondary type

• [docs] Documentation changes
• [test] Adding missing or correcting existing tests

Does this PR introduce a BREAKING CHANGE?

No.

Related issues and/or pull requests

Describe the changes you're proposing

• In v3002.5 (and probably other patched versions), the fix for CVE-2021-25283 enables Jinja2 safe mode, which breaks use of the 'dict' in x.__class__.__name__ workaround
• Workaround no longer needed as CentOS 6 is EOL

Pillar / config required to test the proposed changes

salt:
  master:
    ext_pillar:
      - cmd_yaml: cat /etc/salt/yaml
      - git:
          - develop https://gitserver/git-pillar.git:
              - env: base
      - reclass:
          inventory_base_uri: /etc/reclass

Debug log showing how the proposed changes work

Unable to manage file: Jinja syntax error: access to attribute '__class__' of 'list' object is unsafe.; line 1206
              
              ---
              [...]
              {%- for pillar in cfg_master['ext_pillar'] -%}
                {%- for key in pillar -%}
                  {%- if pillar[key] is string %}
                - {{ key }}: {{ pillar[key] }}
                  {#- Workaround for missing `is mapping` on CentOS 6, see #193: #}
                  {%- elif pillar[key] is iterable and 'dict' not in pillar[key].__class__.__name__ %}    <======================
                - {{ key }}:
                    {%- for parameter in pillar[key] %}
                      {%- if parameter is iterable and parameter is not string %}
                      {%- for param, children in parameter.items() %}
                  - {{ param }}:
              [...]
              ---

Documentation checklist

• Updated the README (e.g. Available states).
• Updated pillar.example.

Testing checklist

• Included in Kitchen (i.e. under state_top).
• Covered by new/existing tests (e.g. InSpec, Serverspec, etc.).
• Updated the relevant test pillar.

Additional context

[myii]

Merged, thanks for the quick fix, @dafyddj.

[saltstack-formulas-travis]

🎉 This PR is included in version 1.7.5 🎉

The release is available on GitHub release

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