[patch] PL/Python is too lossy with floats

PL/Python uses str(v) to convert float data, but is lossy
by design.  Only repr(v) is guaranteed to have enough
precision to make floats roundtrip properly:

  https://docs.python.org/2/library/functions.html#func-repr
  https://docs.python.org/2/library/functions.html#str

Example:

  $ python
  >>> repr(100100100.654321)
  '100100100.654321'
  >>> str(100100100.654321)
  '100100100.654'

Attached patch uses PyObject_Repr() for float data.

As it's annoying-to-debug problem and the patch is simple,
perhaps it's worth backporting?

--
marko