changed flask cors import

[rveachkc]

Per the Flask Documentation:

Extension imports

Extension imports of the form flask.ext.foo are deprecated, you should use flask_foo.

The old form still works, but Flask will issue a flask.exthook.ExtDeprecationWarning for each extension you import the old way. We also provide a migration utility called flask-ext-migrate that is supposed to automatically rewrite your imports for this.

This changes the CORS import from the deprecated form to the preferred form.

Issue #2979

[sampsyo]

Awesome; thank you! We should indeed be using the new package name. Nice work! ✨