fix: revert to using is mapping in Jinja2

* CVE-2021-25283 enables Jinja2 safe mode, which breaks use of
  `'dict' in x.__class__.__name__` workaround
* Workaround no longer needed as CentOS 6 is EOL

test/integration/v3000-py2/files/_mapdata/ubuntu-16.yaml

@@ -45,6 +45,13 @@ values:
     key_url: https://repo.saltstack.com/apt/ubuntu/16.04/amd64/3000/SALTSTACK-GPG-KEY.pub
     libgit2: libgit2-22
     master:
+      ext_pillar:
+      - cmd_yaml: cat /etc/salt/yaml
+      - stack:
+          - /path/to/stack1.cfg
+          - /path/to/stack2.cfg
+      - reclass:
+          inventory_base_uri: /etc/reclass
       file_roots:
         base:
         - /srv/salt

test/integration/v3000-py2/files/_mapdata/ubuntu-18.yaml

@@ -45,6 +45,13 @@ values:
     key_url: https://repo.saltstack.com/apt/ubuntu/18.04/amd64/3000/SALTSTACK-GPG-KEY.pub
     libgit2: libgit2-22
     master:
+      ext_pillar:
+      - cmd_yaml: cat /etc/salt/yaml
+      - stack:
+          - /path/to/stack1.cfg
+          - /path/to/stack2.cfg
+      - reclass:
+          inventory_base_uri: /etc/reclass
       file_roots:
         base:
         - /srv/salt