Unit tests fail due to unsupported .removeprefix() in python 3.7

Description
===========
When running unit tests on python3.7 two tests fail:
nova.tests.unit.console.test_websocketproxy.NovaProxyRequestHandlerBaseTestCase.test_reject_open_redirect
nova.tests.unit.console.test_websocketproxy.NovaProxyRequestHandlerBaseTestCase.test_reject_open_redirect_3_slashes

Steps to reproduce
==================
Checkout to latest stable/train as of 17.08.2023
$ tox -e py37

Expected result
=============
All unit tests pass

Actual result
=============
`Captured traceback:
~~~~~~~~~~~~~~~~~~~
    b'Traceback (most recent call last):'
    b' File "/builds/nfv-platform/nova/nova/tests/unit/console/test_websocketproxy.py", line 678, in test_reject_open_redirect'
    b" location = location.removeprefix('Location: ').rstrip('\\r\\n')"
    b"AttributeError: 'str' object has no attribute 'removeprefix'"
    b''`

`Captured traceback:
~~~~~~~~~~~~~~~~~~~
    b'Traceback (most recent call last):'
    b' File "/builds/nfv-platform/nova/nova/tests/unit/console/test_websocketproxy.py", line 685, in test_reject_open_redirect_3_slashes'
    b" self.test_reject_open_redirect(url='///example.com/%2F..')"
    b' File "/builds/nfv-platform/nova/nova/tests/unit/console/test_websocketproxy.py", line 678, in test_reject_open_redirect'
    b" location = location.removeprefix('Location: ').rstrip('\\r\\n')"
    b"AttributeError: 'str' object has no attribute 'removeprefix'"
    b''`

Environment
===========
Python version 3.7.17