Update testing to Python 3.9 and linters

In preparation for an upcoming release, add testing for latest
Python (3.9). Switch tox to use Python 3 by default, and rename the
testenv for flake8 from pep8 to linters, consistent with other tools
and libraries OpenDev maintains. Update to a newer hacking plugin,
which will use newer flake8 as well. Ignore rules about line breaks
around comparison operators, as well as those related to ambiguous
variable names, at least for now. Also build distribution artifacts
on a more recent platform so we get newer Setuptools with support
for the latest package metadata, in preparation for a coming change
to update that.

Change-Id: I2130d66fc9aadaa9fe09635b59475be71938132e

.zuul.yaml

@@ -60,19 +60,25 @@
       - publish-opendev-tox-docs
     check:
       jobs:
-        - tox-pep8
+        - tox-linters
         - tox-py27
         - tox-py35:
             nodeset: ubuntu-xenial
-        - build-python-release
+        - tox-py39:
+            nodeset: ubuntu-focal
+        - build-python-release:
+            nodeset: ubuntu-focal
         - gear-build-opendev-image
     gate:
       jobs:
-        - tox-pep8
+        - tox-linters
         - tox-py27
         - tox-py35:
             nodeset: ubuntu-xenial
-        - build-python-release
+        - tox-py39:
+            nodeset: ubuntu-focal
+        - build-python-release:
+            nodeset: ubuntu-focal
         - gear-upload-opendev-image
     promote:
       jobs:
@@ -80,7 +86,8 @@
         - gear-promote-opendev-image
     release:
       jobs:
-        - opendev-release-python
+        - opendev-release-python:
+            nodeset: ubuntu-focal
         - upload-docker-image:
             secrets:
               name: docker_credentials

test-requirements.txt

@@ -1,4 +1,4 @@
-hacking>=0.12.0,!=0.13.0,<0.14  # Apache-2.0
+hacking>=2.0.0,<2.1.0  # Apache-2.0
 
 coverage>=3.6
 fixtures>=0.3.12

tox.ini

@@ -1,11 +1,13 @@
 [tox]
 minversion = 1.6
 skipsdist = True
-envlist = py34,py35,py27,pep8
+envlist = py3,py27,linters
+ignore_basepython_conflict = true
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
+basepython = python3
 install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
@@ -15,7 +17,7 @@ commands =
 [tox:jenkins]
 sitepackages = True
 
-[testenv:pep8]
+[testenv:linters]
 commands = flake8
 
 [testenv:cover]
@@ -30,7 +32,7 @@ commands = {posargs}
 exclude = .venv,.tox,dist,doc,*.egg
 show-source = true
 # E123, E125, E129 and H ignored intentionally in this code-base
-ignore = E123,E125,E129,H
+ignore = E123,E125,E129,E741,W504,H
 
 [testenv:docs]
 basepython = python3