fix: Add W503 to flake8 extend-ignore

For some reason flake8 is now throwing W503, which should be disabled by
default, adding to extend-ignore fixes it, so doing that for now.

tox.ini

@@ -19,4 +19,4 @@ deps =
 exclude = .tox,env,build,cocoalib,cocoa,help,./qt/dg_rc.py,cocoa/run_template.py,./pkg
 max-line-length = 120
 select = C,E,F,W,B,B950
-extend-ignore = E203, E501
+extend-ignore = E203, E501, W503