[mypy] - ignore erroneous type warning in new imageio version.

[aclegg3]

Motivation and Context

New imageio version may have conflicting API. mypy asserts that format should be Format type, but should actually be string as written.

How Has This Been Tested

CI SSH and local

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.