More cleanup. Moved all Unicode and str/bytes info to the text/data

subsection in common pitfalls, rather than spreading it out.  Made
this the last subsection of common pitfalls.  All XXX comments are now
gone.  I'm sure much is still missing, we'll have to clean that up
post 3.0.  At least all PEPs and all implemented items in PEP 3100
have at least one mention.