fixes #1427

changes in f524bcc caused assignments in nested loops to still be referenced after the end of the iteration. now, all variables assigned in loops will be cleared from the top level so that jinja won't attempt to resolve them outside of the loop.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.