Skip flake8 bits on Windows

since pip isn't already there, installing it is a bit of a pain, and the results
*should* be the same on all platforms

appveyor.yml

@@ -18,10 +18,8 @@ install:
 - go env
 - go get github.com/urfave/gfmxr/...
 - go get -v -t ./...
-- pip install --user flake8
 
 build_script:
-- flake8 runtests cli-v1-to-v2
 - python runtests vet
 - python runtests test
 - python runtests gfmxr