Allow shell.py to emit its prompt multiple times

1d0f94c allowed shell.py to emit a single '>>> ' on stdout.
Sometimes it emits '>>> >>> ', so allow that too.
  https://travis-ci.org/wikimedia/pywikibot-core/jobs/43425062

Change-Id: I746187d58e254d2d83b41b24874479bb954475b4
Merge "Allow shell.py to emit its prompt multiple times"