Change cwd test to use a tempdir

Many system users will not have a homedir, or will
have this value set to `/dev/null`, which will break this
test.  Instead we'll just create a tempdir and cd to that instead.

Actual test fix is in [test_cwd_doesnt_matter](https://github.com/jamesls/aws-cli/compare/aws:develop...jamesls:skip-if-dev-null?expand=1#diff-ee8189eeaaa621c78b2559200247ea23R1315)
Also includes an additional commit for flake8 cleanup (https://github.com/jamesls/aws-cli/commit/a6f10fdba5594910432c81aad17ed70947528912)


cc @kyleknap