Use an older version of pip for py35 old deps.

.buildkite/scripts/test_old_deps.sh

@@ -10,4 +10,7 @@ apt-get install -y python3.5 python3.5-dev python3-pip libxml2-dev libxslt-dev x
 
 export LANG="C.UTF-8"
 
+# Prevent virtualenv from auto-updating pip to an incompatible version
+export VIRTUALENV_NO_DOWNLOAD=1
+
 exec tox -e py35-old,combine

changelog.d/9217.misc

@@ -0,0 +1 @@
+Fix the Python 3.5 old dependencies build.

tox.ini

@@ -18,11 +18,13 @@ deps =
     # installed on that).
     #
     # anyway, make sure that we have a recent enough setuptools.
-    setuptools>=18.5
+    setuptools>=18.5 ; python_version >= '3.6'
+    setuptools>=18.5,<51.0.0 ; python_version < '3.6'
 
     # we also need a semi-recent version of pip, because old ones fail to
     # install the "enum34" dependency of cryptography.
-    pip>=10
+    pip>=10 ; python_version >= '3.6'
+    pip>=10,<21.0 ; python_version < '3.6'
 
 # directories/files we run the linters on.
 # if you update this list, make sure to do the same in scripts-dev/lint.sh
@@ -104,15 +106,10 @@ usedevelop=true
 [testenv:py35-old]
 skip_install=True
 deps =
-    # Ensure a version of setuptools that supports Python 3.5 is installed.
-    setuptools < 51.0.0
-
     # Old automat version for Twisted
     Automat == 0.3.0
-
     lxml
-    coverage
-    coverage-enable-subprocess==1.0
+    {[base]deps}
 
 commands =
     # Make all greater-thans equals so we test the oldest version of our direct
@@ -169,6 +166,8 @@ commands = {toxinidir}/scripts-dev/generate_sample_config --check
 skip_install = True
 deps =
     coverage
+    pip>=10 ; python_version >= '3.6'
+    pip>=10,<21.0 ; python_version < '3.6'
 commands=
     coverage combine
     coverage report