Introducing auth.DELETE_ATTRIBUTE sentinel value

[hiranya911]

We allow deleting some user attributes (display_name, photo_url etc) by explicitly passing None to the update_user() function. To make this API more idiomatic, we wish to introduce an auth.DELETE_ATTRIBUTE sentinel value (constant) instead.

Before:

# Remove the photo_url from the user account
auth.update_user(uid, photo_url=None)

After:

# Remove the photo_url from the user account
auth.update_user(uid, photo_url=auth.DELETE_ATTRIBUTE)

We will support both None and the new sentinel value for now. We will stop supporting None in the major release.

go/firebase-python-sentinels