Remove Python 3 only code within a Python 2 only block

The whole function is surrounded by a ``#if PY_MAJOR_VERSION < 3``
so there is no point keeping ``#if (PY_MAJOR_VERSION >= 3`` blocks
in its code.
Fix parse of times with only milliseconds in the Python impl

Also, recognize also a space as separator between the date and the
time, for consistency with the C implementation.
Comply with Bob's requirements expressed in issue #86

* rename the flag to activate the new behaviour from "handle_datetime"
  to a more clear "iso_datetime"

* serialize datetimes and times with an ending "Z" to make it explicit
  that they are naive values; the parser does recognize either forms
  though

Correct also the Python 3 variant of the C speedups to properly handle
the different FSR internal representations.