tests/ci: use flake8 --extend-ignore instead of --ignore (#5550)

Using --ignore completely overrides the ignore list, while
--extend-ignore (introduced in flake8 v.3.6) extends the default
settings.

Overriding the built-in defaults seems to enable both W503 and W504,
which are impossible to satisfy at the same time.

setup.cfg

@@ -173,4 +173,4 @@ exclude =
    bootloader
 show-source = True
 # E265 - block comment should start with '# '
-ignore = E265
+extend-ignore = E265