Reland "Port _FileDescriptorStreamsNonBlocking to use poll()"

Now that repo 2 requires Python 3, we can reland this.

This reverts commit 91d9587e.

Change-Id: Id5b178ebb53bdba04bfa79cbb5c698ae5080c957
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/258672


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Theodore Dubois <tbodt@google.com>