Add hidden=True to dash pages callback

[l0uden]

Added hidden=True to the dash pages callbacks.
Now this callbacks will not appear in the debug mode callback diagram.

Solves this issue: #3463

Contributor Checklist

• I have run the tests locally and they passed. (refer to testing section in contributing)

[l0uden]

All failed tests have the same error:

INTERNALERROR>     warnings.warn(
INTERNALERROR> pytest.PytestRemovedIn9Warning: The (startdir: py.path.local) argument is deprecated, please use (start_path: pathlib.Path)
INTERNALERROR> see https://docs.pytest.org/en/latest/deprecations.html#py-path-local-arguments-for-hooks-replaced-with-pathlib-path

It can be fixed by lowering pytest version or by changing the tests code.

[AnnMarieW]

Hi @l0uden

That error when running the tests will be fixed by #3541 🎉