Update flake8 line-length to match black's default line length

setup.cfg

@@ -34,7 +34,7 @@ use_parentheses=true
 ignore = E203, E712, E711, W503
 select = C,E,F,W,B,B9
 max-complexity = 10
-max-line-length = 80
+max-line-length = 88
 exclude =
     # allowed to break the rules
     flaskbb/configs/default.py,