test_config: python3 fix
test_config: python3 fix

1) use io for StringIO in python3
2) use builtins instead of __builtin__
3) use StringIO instead of BytesIO when mocking aws config
Othervise this test fails on python3.

Signed-off-by: Ruslan Kuprieiev <f6538312ce47017d734b1357aabc970ebf896e32@gmail.com>

dataversioncontrol/dvc,efiop/dvc,dmpetrov/dataversioncontrol,ophiry/dvc,dmpetrov/dataversioncontrol,efiop/dvc,dataversioncontrol/dvc,ophiry/dvc