Fix CI error reported by ansible-lint

[Perdu]

SUMMARY

As discussed in #898, the CI has errors reported by ansible-lint:
https://dev.azure.com/ansible/community.postgresql/_build/results?buildId=165038&view=logs&j=df593170-0476-56b6-4a33-d8e2815d43d8&t=ed5b4094-b5e7-57c7-f236-602faf856c9f

01:27 ERROR: Found 9 pylint issue(s) which need to be resolved:
01:27 ERROR: plugins/module_utils/postgres.py:22:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (50%)
01:27 ERROR: plugins/modules/postgresql_copy.py:184:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (0%)
01:27 ERROR: plugins/modules/postgresql_info.py:159:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (50%)
01:27 ERROR: plugins/modules/postgresql_publication.py:257:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (0%)
01:27 ERROR: plugins/modules/postgresql_query.py:269:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (50%)
01:27 ERROR: plugins/modules/postgresql_script.py:219:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (50%)
01:27 ERROR: plugins/modules/postgresql_subscription.py:217:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (0%)
01:27 ERROR: plugins/modules/postgresql_tablespace.py:195:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (0%)
01:27 ERROR: plugins/modules/postgresql_user_obj_stat_info.py:110:0: ansible-bad-import-from: Import iteritems from the Python standard library equivalent instead of ansible.module_utils.six (0%)

This patch fixes these errors by removing the wrong imports and using .items() instead of iteritems()

ISSUE TYPE

• Bugfix Pull Request

COMPONENT NAME

It's in a lot of modules

ADDITIONAL INFORMATION

(I'm don't have a local full test environment for this project and could not reproduce the issue locally. I'm trusting the CI to test this properly)

[Perdu]

This did pass the failing test. The 2 remaining ones are unrelated.

[Andersson007]

@Perdu thanks for the contribution! CI is green again. This is that kind of contributions this project needs!
@hunleyd thanks for reviewing and merging!