compatibility update for Python 3.6

src/bits.py

@@ -933,6 +933,8 @@ class Bytes:
     A colletion of Bits with convenient properties for working with binary data
     """
 
+    import sys
+
     def __init__(self, var=None, byteorder="big"):
         """
         var: a supported variant (object)
@@ -1135,10 +1137,23 @@ class Bytes:
         """
         Return the hex-string representation of self
         """
+        hexvalue = bytes(self).hex()
         if sep is None or bytes_per_sep is None or bytes_per_sep == 0:
-            return bytes(self).hex()
+            return hexvalue
         else:
-            return bytes(self).hex(sep=sep, bytes_per_sep=bytes_per_sep)
+            if bytes_per_sep > 1:
+                hexvalue = hexvalue[::-1] # reverse the hex string
+                sep = sep[::-1]
+            i = 0
+            retvalue = ""
+            while i < len(hexvalue):
+                if i > 0:
+                    retvalue = retvalue + sep
+                retvalue = retvalue + hexvalue[i:(abs(bytes_per_sep * 2) + i)]
+                i += abs(bytes_per_sep * 2)
+            if bytes_per_sep > 1:
+                retvalue = retvalue[::-1]
+            return retvalue
 
     @property
     def bytes(self):