Bump flake8 line length to 99

Change-Id: Ib69f2100a7097424465808b670513892a838851e
diff --git a/tox.ini b/tox.ini
index f0c9589..2cb7d3b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -149,3 +149,4 @@
 [flake8]
 # Ignore E902 because the unit_tests directory is missing in the built charm.
 ignore = E402,E226,W503,W504,E902
+max-line-length = 100