diff --git a/azure-pipelines.yml b/azure-pipelines.yml
index 9a262b3..f99c008 100644
--- a/azure-pipelines.yml
+++ b/azure-pipelines.yml
@@ -16,5 +16,5 @@ jobs:
 - template: job--pre-commit.yml@asottile
 - template: job--python-tox.yml@asottile
   parameters:
-    toxenvs: [py36, py37, py38]
+    toxenvs: [py36, py37, py38, py39]
     os: linux
diff --git a/tests/features/conftest.py b/tests/features/conftest.py
index 73c4d71..b9630b8 100644
--- a/tests/features/conftest.py
+++ b/tests/features/conftest.py
@@ -391,6 +391,7 @@ class DeferredRunner:
 
     _curses_cbreak = _curses_endwin = _curses_noecho = _curses__noop
     _curses_nonl = _curses_raw = _curses_use_default_colors = _curses__noop
+    _curses_set_escdelay = _curses__noop
 
     _curses_error = curses.error  # so we don't mock the exception