I rebased the PR to fix a merge conflict.

I addressed@eryksun's and@JelleZijlstra's reviews: thanks!

@serhiy-storchaka@erlend-aasland: Would you mind to review this change?

@serhiy-storchaka: I addressed your review. Would you mind to review my updated PR?

What about os.environb.refresh()? Does it exist? Does it work?

I added a test on os.environb.refresh(). Yes, it works as expected :-)

Should os.environb.refresh() be documented? We perhaps do not want to attract too much attention to it, as its effect is the same as of os.environ.refresh().

Hum, I prefer to not document it athttps://docs.python.org/dev/library/os.html#os.environb :-)

(Ooops, I closed on [Close] instead of [Comment], sorry.)

@serhiy-storchaka: PR updated for os.unsetenv().

Oh, I didn't know that Python has two functions (putenv, unsetenv) which modify directly the environment without updatingos.environ.