CI: Limit to Tox < 4.0 to avoid plugin incompatibility

I'm abandoning tox-factor because having any [tox] requires = ... causes tox
3.x to create an isolated virtualenv for running tox itself. Since Tox 4.x was
released that virtualenv gets it, which is incompatible with the tox-factor
plugin.
e.g.
```
Traceback (most recent call last):
File
"/Users/runner/work/1/s/.tox/.tox/lib/python3.10/site-packages/tox_factor/compat.py",
line 2, in <module>
    from tox.config.parallel import ENV_VAR_KEY_PUBLIC as TOX_PARALLEL_ENV
ModuleNotFoundError: No module named 'tox.config.parallel'
```

.ci/azure-pipelines-steps.yml

@@ -1,3 +1,10 @@
+# Each step entry runs a task (Azure Pipelines analog of an Ansible module).
+# https://learn.microsoft.com/en-us/azure/devops/pipelines/tasks/reference/?view=azure-pipelines&viewFallbackFrom=azure-devops#tool
+
+# `{script: ...}` is shorthand for `{task: CmdLine@<mumble>, inputs: {script: ...}}`.
+# https://learn.microsoft.com/en-us/azure/devops/pipelines/yaml-schema/steps-script?view=azure-pipelines
+# https://learn.microsoft.com/en-us/azure/devops/pipelines/tasks/reference/cmd-line-v2?view=azure-pipelines
+
 steps:
 - task: UsePythonVersion@0
   displayName: Install python
@@ -11,7 +18,7 @@ steps:
   displayName: Install build deps
   condition: and(eq(variables['python.version'], ''), eq(variables['Agent.OS'], 'Linux'))
 
-- script: python -mpip install tox
+- script: python -mpip install "tox<4.0"
   displayName: Install tooling
 
 - script: python -mtox -e "$(tox.env)"

tox.ini

@@ -42,8 +42,6 @@ envlist =
     py{27,36,310}-mode_mitogen-distro_debian{9,10,11},
     py{27,36,310}-mode_mitogen-distro_ubuntu{1604,1804,2004},
     report,
-requires =
-    tox-factor
 
 [testenv]
 basepython =