Add highlighting Python operators (bug#59629)

Add highlighting Python operators (bug#59629)

* lisp/progmodes/python.el (python-rx): Remove "is" and
"not" from operator.
(python-font-lock-keywords-maximum-decoration): Add
highlighting operators.
* test/lisp/progmodes/python-tests.el
(python-font-lock-assignment-statement-1)
(python-font-lock-assignment-statement-2)
(python-font-lock-assignment-statement-3)
(python-font-lock-assignment-statement-4)
(python-font-lock-assignment-statement-5)
(python-font-lock-assignment-statement-6)
(python-font-lock-assignment-statement-7)
(python-font-lock-assignment-statement-8)
(python-font-lock-assignment-statement-9)
(python-font-lock-assignment-statement-10)
(python-font-lock-assignment-statement-11)
(python-font-lock-assignment-statement-12)
(python-font-lock-assignment-statement-13)
(python-font-lock-assignment-statement-14)
(python-font-lock-assignment-statement-15)
(python-font-lock-assignment-statement-16)
(python-font-lock-assignment-statement-17)
(python-font-lock-assignment-statement-18)
(python-font-lock-assignment-statement-multiline-1)
(python-font-lock-assignment-statement-multiline-2)
(python-font-lock-assignment-statement-multiline-3)
(python-font-lock-assignment-statement-multiline-4)
(python-font-lock-assignment-statement-multiline-5)
(python-font-lock-assignment-statement-multiline-6)
(python-font-lock-escape-sequence-multiline-string): Modify
expectations to include faces for operators.
(python-font-lock-operator-1, python-font-lock-operator-2): New tests.