tox.ini: flake8 --max-line-length=88

Aligns with https://black.readthedocs.io/en/stable/the_black_code_style.html#line-length
diff --git a/tox.ini b/tox.ini
index b87e5f1..004c01d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -19,7 +19,7 @@
     flake8
     flake8-docstrings
 commands =
-    flake8
+    flake8 --max-line-length=88
 
 [testenv:release]
 deps =