diff --git a/testkit/_common.py b/testkit/_common.py
index d4f8da965..1ab0254ed 100644
--- a/testkit/_common.py
+++ b/testkit/_common.py
@@ -13,5 +13,9 @@ def run(args, env=None):
     )
 
 
-def run_python(args, env=None):
-    run([TEST_BACKEND_VERSION, "-u", "-W", "error", *args], env=env)
+def run_python(args, env=None, warning_as_error=True):
+    cmd = [TEST_BACKEND_VERSION, "-u"]
+    if warning_as_error:
+        cmd += ["-W", "error"]
+    cmd += list(args)
+    run(cmd, env=env)
diff --git a/testkit/build.py b/testkit/build.py
index dd1f318bc..2937f942e 100644
--- a/testkit/build.py
+++ b/testkit/build.py
@@ -9,6 +9,8 @@
 
 if __name__ == "__main__":
     run_python(["setup.py", "build"])
-    run_python(["-m", "pip", "install", "-U", "pip"])
-    run_python(["-m", "pip", "install", "-Ur",
-                "testkitbackend/requirements.txt"])
+    run_python(["-m", "pip", "install", "-U", "pip"], warning_as_error=False)
+    run_python(
+        ["-m", "pip", "install", "-Ur", "testkitbackend/requirements.txt"],
+        warning_as_error=False
+    )