Now that Python 2 is not supported, the bytes/str boundaries are more
clear and explicit. Using six.ensure_* methods for backwards
compatibility is no longer necessary as the types are known and verified
using mypy.

One exception is tests/lib/wheel.py which allows tests to pass test
setup data as either bytes or str.

The module operations.install.wheel also remains untouched as it is
especially delicate to bytes/str mixups and the current version is
working.