[py] fix typing in get_full_page_screenshot_as_png

[eNcacz]

Description

Fix for issue 11157

Motivation and Context

Wrong type for get_full_page_screenshot_as_png method causes false positive mypy findings.

Types of changes

• 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

• I have read the contributing document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Type for returned value was changed from str to bytes to reflect the reality.

[titusfortner]

Thanks!