diff --git a/newsfragments/3375.minor b/newsfragments/3375.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/tox.ini b/tox.ini
index 15cafe197..0376ab28e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -210,7 +210,9 @@ extras =
 deps =
     {[testenv]deps}
     packaging
-    pyinstaller
+    # PyInstaller 4.0 drops Python 2 support.  When we finish porting to
+    # Python 3 we can reconsider this constraint.
+    pyinstaller < 4.0
 # Setting PYTHONHASHSEED to a known value assists with reproducible builds.
 # See https://pyinstaller.readthedocs.io/en/stable/advanced-topics.html#creating-a-reproducible-build
 setenv=PYTHONHASHSEED=1