Bug in vchar_or_obs_text regex: shouldn't accept DEL #58

Currently our vchar_or_obs_text regex is:

vchar_or_obs_text = r"[\x21-\xff]"

However, this is incorrect... VCHAR is %x21-7E and obs-text is %x80-FF, so there's actually a one character gap in the middle of the range: \x7f (DEL) should not be included.

Of course, we might need to relax these checks anyway (#57), but if we do this should be intentional, not an accident :-)