Move flake8 config into setup.cfg

At the moment the two are duplicative, and it’s not clear which takes
precedence.

We need to keep the `excludes` line otherwise running the `flake8`
command takes ~3 times longer and may lint 3rd party files that we don’t
control.

.flake8

@@ -1,7 +0,0 @@
-[flake8]
-# Rule definitions: http://flake8.pycqa.org/en/latest/user/error-codes.html
-# W503: line break before binary operator
-exclude = venv*,__pycache__,node_modules,cache,migrations,build
-ignore = W503
-max-complexity = 14
-max-line-length = 120

setup.cfg

@@ -3,6 +3,7 @@ xfail_strict=true
 
 
 [flake8]
+exclude = venv*,__pycache__,node_modules,cache,migrations,build
 # W504 line break after binary operator
 extend_ignore=B306, W504
 select=B901