build: Add flake8-py39

This is now available in CI.

Bug: T270389
Change-Id: I193f5df5943c42496cefee56c7c6e091b855f4f2
---
M tox.ini
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tox.ini b/tox.ini
index 6d7c8ec..5d25e64 100644
--- a/tox.ini
+++ b/tox.ini
@@ -5,7 +5,7 @@
 skip_missing_interpreters = True
 envlist =
     commit-message
-    flake8-{py38,py36}
+    flake8-{py39,py38,py36}
     hacking-py37
 
 [params]
@@ -21,6 +21,7 @@
     py36: python3.6
     py37: python3.7
     py38: python3.8
+    py39: python3.9
 setenv =
     VIRTUAL_ENV={envdir}
     PYWIKIBOT_NO_USER_CONFIG=2
@@ -30,6 +31,7 @@
     flake8: flake8 --version
     flake8-py36: flake8 --doctests {posargs} {[params]exclude}
     flake8-py38: flake8 --doctests {posargs}
+    flake8-py39: flake8 --doctests {posargs}
 
     hacking: flake8 --version
     hacking: flake8 --format=default {posargs}

