test: add unit tests for relative_import

[eeshsaxena]

Description

_plotly_utils.importers.relative_import is the helper behind plotly's lazy
submodule imports - it returns the (__all__, __getattr__, __dir__) triple a
package assigns to enable PEP 562 lazy loading. It had no test coverage.

This adds tests/test_core/test_utils/test_importers.py covering:

• __all__ listing the leaf names of the requested modules and classes
• __getattr__ lazily importing a submodule
• __getattr__ lazily importing a class off a submodule
• __getattr__ raising AttributeError for an unknown name
• __dir__ returning __all__

The tests use the stdlib collections package as a stable lazy-import target,
so they don't depend on plotly's own module layout. No source changes.

Verification

pytest tests/test_core/test_utils/test_importers.py - 5 passed. ruff check
/ ruff format --check clean.