ENH: add tobytes and stop using tostring in documentation

tostring returns bytes which are not equal to string, so provide a
tobytes function alias.
tostring does not emit a deprecation warning yet so rdepends do not need
to check two names to support older versions of numpy without warnings.
Merge pull request #4257 from juliantaylor/tostring-depr

ENH: add tobytes and stop using tostring in documentation