PY3: Use `enter_context` instead of `push` for `ExitStack`
PY3: Use `enter_context` instead of `push` for `ExitStack`

`salt.utils.context.ChildContextDict.__enter__()` was not being
invoked in Python 3 because `ExitStack.push()` only ensures that
`__exit__` is invoked. Switch to `ExitStack.enter_context()` which
is more appropriate.

Signed-off-by: Sergey Kizunov <dfddabaa9af05982b01940b837a87844148069c9@ni.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt