I'd like to reopen this. I'm still in favor of something like to this
algorithm:

def float_repr(x):
  s = "%.16g" % x
  if float(s) != x:
    s = "%.17g" % x
  s1 = s
  if s1.startswith('-'):
    s1 = s[1:]
  if s1.isdigit():
    s += '.0'  # Flag it as a float
  # XXX special case inf and nan, see floatobject.c::format_double
  return s

combined with explicitly using %.17g or the new hex notation (see issue
3008) in places where cross-platform correctness matters, like pickle
and marshal.

This will ensure float(repr(x)) == x on all platforms, but not cross
platforms -- and I don't care.

I'm fine with only doing this in 3.0.