Switch from tox to nox

Recent tox releases have put us on a config treadmill. Avoid these
issues entirely by using nox. Nox is a tox alternative that uses
standard tools like pip and should be simpler to use for us.

Change-Id: Ifce288808f535b5ee071dddc597ef54a6d3d03bb
Depends-On: https://review.opendev.org/883664
diff --git a/.zuul.yaml b/.zuul.yaml
index 6107d2f..0e78370 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -2,8 +2,8 @@
     check:
       jobs:
         - gerritlib-jeepyb-integration
-        - tox-pep8
+        - nox-linters
     gate:
       jobs:
         - gerritlib-jeepyb-integration
-        - tox-pep8
+        - nox-linters