[python] h2o.no_progress as a contextmanager

[hutch3232]

Is your feature request related to a problem? Please describe.
I'd love to be able temporarily disable the progress bar while running chunks of h2o code using with.

Describe the solution you'd like
I think the contextmanager decorator would solve this.

Describe alternatives you've considered
I could write a try finally block.

Additional context
Looks like R can do this based on #6991

[tomasfryda]

We already have internal no_progress_block (see h2o.explanation._explain.no_progress_block) which is a context manager. Changing no_progress to context manager was couple years ago considered but we didn't want to change API so we added a new function that works as a context manager.

I'll keep this issue open until we export the no_progress_block so it's not so hidden.