CI: Workaround "No module named 'setuptools.command.test'"

Pip 72 was released yesterday (2024-07-28), dropping `setup.py test` support.
hdrhistogram 0.6.1 requires it to install.

For now constrain Pip to earlier releases, so our tests can be run.

refs #1090

tests/ansible/requirements.txt

@@ -1,4 +1,7 @@
 paramiko==2.3.2  # Last 2.6-compat version.
+# Incompatible with pip >= 72, due to removal of `setup.py test`:
+#   ModuleNotFoundError: No module named 'setuptools.command.test'
+# https://github.com/pypa/setuptools/issues/4519
 hdrhistogram==0.6.1
 PyYAML==3.11; python_version < '2.7'
 PyYAML==5.3.1; python_version >= '2.7'  # Latest release (Jan 2021)

tests/constraints.txt

@@ -0,0 +1,3 @@
+# Setuptools 72 removed `setup.py test`. hdrhistogram 0.6.1 still depends on it.
+# TODO Bump dependencies and unconstrain Pip.
+setuptools<72

tox.ini

@@ -109,6 +109,7 @@ setenv =
     ANSIBLE_STRATEGY = mitogen_linear
     NOCOVERAGE_ERASE = 1
     NOCOVERAGE_REPORT = 1
+    PIP_CONSTRAINT={toxinidir}/tests/constraints.txt
     # Only applicable to MODE=mitogen
     distro_centos5: DISTRO=centos5
     distro_centos6: DISTRO=centos6