ci: Fix Python 2.7 builds on macOS 11

With current macOS 11 runner images (20231216.1) the `python` on `$PATH` is
Python 3.12 and setuptools isn't installed by default. E.g.

```
python -mtox -e "py27-mode_localhost-ansible4"
========================== Starting Command Output ===========================
/bin/bash --noprofile --norc
/Users/runner/work/_temp/93a29c4c-f606-45e4-8dbd-a4a5f51b8730.sh
GLOB sdist-make: /Users/runner/work/1/s/setup.py
ERROR: invocation failed (exit code 1), logfile:
/Users/runner/work/1/s/.tox/log/GLOB-0.log
================================== log start
===================================
Traceback (most recent call last):
  File "/Users/runner/work/1/s/setup.py", line 32, in <module>
    from setuptools import find_packages, setup
ModuleNotFoundError: No module named 'setuptools'
```

Installing setuptools under Python 3.12 chooses package versions incompatible
with Python 2.7. Additionally Mitogen isn't yet compatible with Python 3.12
(mitogen-hq#1033), so tests that call a local context with `python` fail.

.ci/azure-pipelines-steps.yml

@@ -13,21 +13,33 @@ steps:
     versionSpec: '$(python.version)'
   condition: ne(variables['python.version'], '')
 
-- script: |
-    type python
-    python --version
-  displayName: Show python version
-
 - script: |
     sudo apt-get update
     sudo apt-get install -y python2-dev python3-pip virtualenv
   displayName: Install build deps
   condition: and(eq(variables['python.version'], ''), eq(variables['Agent.OS'], 'Linux'))
 
-- script: python -mpip install "tox<4.0"
+- script: |
+    type python && python --version
+    type python2 && python2 --version
+    type python3 && python3 --version
+  displayName: Show python versions
+
+- script: |
+    if [[ $(uname) == "Darwin" ]]; then
+        python2 -m ensurepip --user --altinstall --no-default-pip
+        python2 -m pip install --user "tox<4.0"
+    else
+        python -m pip install "tox<4.0"
+    fi
   displayName: Install tooling
 
-- script: python -mtox -e "$(tox.env)"
+- script: |
+    if [[ $(uname) == "Darwin" ]]; then
+        python2 -m tox -e "$(tox.env)"
+    else
+        python -m tox -e "$(tox.env)"
+    fi
   displayName: "Run tests"
   env:
     AWS_ACCESS_KEY_ID: $(AWS_ACCESS_KEY_ID)