mkicon.py: Work around Python 3's round() semantics.

Python 3 changed the built-in round() function to use round-to-even for
exact half-way cases.  While this eliminates a bias away from zero and
is thus a better choice in many cases, it's not what mkicon.py expected.

Shadow the built-in with an invocation of the decimal module instead:
this produces identical results for mkicon.py's purposes on both Python
2 and 3.

icons/mkicon.py

@@ -2,6 +2,7 @@
 
 from __future__ import division
 
+import decimal
 import math
 
 # Python code which draws the PuTTY icon components at a range of
@@ -15,6 +16,11 @@ import math
 #
 #  - can we integrate the Mac icons into all this? Do we want to?
 
+# Python 3 prefers round-to-even.  Emulate Python 2's behaviour instead.
+def round(number):
+    return float(
+        decimal.Decimal(number).to_integral(rounding=decimal.ROUND_HALF_UP))
+
 def pixel(x, y, colour, canvas):
     canvas[(int(x),int(y))] = colour