[#24] Add logic to only convert strings to bytes in python3

Adds additional logic statement to check python version. This is
necessary as I couldn't find a way to make the underlaying
functionality compatible with python2 and 3 due to the differences
in the way both versions handle strings - i.e. strings in python2
are treated as bytes objects by default, so converting to bytes is
not necessary in the first place.

iati/core/data.py

@@ -1,4 +1,5 @@
 """A module containing a core representation of an IATI Dataset."""
+import sys
 from lxml import etree
 import iati.core.exceptions
 import iati.core.utilities
@@ -79,7 +80,8 @@ def xml_str(self, value):
                 value_stripped = value.strip()
 
                 # Convert the input to bytes, as etree.fromstring works most consistently with bytes objects, especially if an XML encoding declaration has been used.
-                if isinstance(value_stripped, str):
+                if (isinstance(value_stripped, str)
+                    and sys.version_info.major > 2):  # Python v2 treats strings as byte objects by default
                     value_stripped_bytes = value_stripped.encode()
                 else:
                     value_stripped_bytes = value_stripped