Remove last remnants of Seq.tostring and deprecate it

I noticed we still haven't officially deprecate Seq.tostring (despite it being replaced by `str` in functionality since Biopython 1.44). I removed most of Seq.tostring usage in #62, but there were still some remnants left (both in code and in the docs). This patch removed all of them and officially deprecates it with a deprecation warning and a note in the DEPRECATED file.