Add test interactions for PP572 with --allow-redefinition

[meows4h]

This aims to resolve issue #7314 by adding test interactions with the walrus operator (:=) and the --allow-redefinition flag. In order to accomplish what the documentation provides for the flag, it also added some stubs to the float fixtures file to help with this.

Changes:

• Added new check-pep572-redefinition.test test file
• Added extra stubs to float.pyi to aid the test file

Fixes #7314