Remove use of removeprefix

This is not supported on Python 3.8 [1]. I have no idea why this was not
failing CI.

[1] https://docs.python.org/3.9/library/stdtypes.html#str.removeprefix

Change-Id: I225e9ced0f75c415b1d2fee05440291e3d8635c0
Signed-off-by: Stephen Finucane <[email protected]>
(cherry picked from commit 3ccf82e)
(cherry picked from commit 2d011ac32c132fa5c4d03f8167c2be3648d5d123)

Author: stephenfin

nova/tests/unit/console/test_websocketproxy.py

@@ -637,7 +637,9 @@ def test_reject_open_redirect(self, url='//example.com/%2F..'):
         # now the same url but with extra leading '/' characters removed.
         if expected_cpython in errmsg:
             location = result[3].decode()
-            location = location.removeprefix('Location: ').rstrip('\r\n')
+            if location.startswith('Location: '):
+                location = location[len('Location: '):]
+            location = location.rstrip('\r\n')
             self.assertTrue(
                 location.startswith('/example.com/%2F..'),
                 msg='Redirect location is not the expected sanitized URL',