SwiftBuildSupport.py: Python 3 compatibility fixes

This change allows me to get `update-checkout` and `build-script`
running with Python 3.

utils/SwiftBuildSupport.py

@@ -10,7 +10,11 @@
 
 from __future__ import print_function
 
-import ConfigParser
+try:
+    import ConfigParser # Python 2
+except ImportError:
+    import configparser as ConfigParser # Python 3
+
 import os
 import pipes
 import subprocess
@@ -132,7 +136,7 @@ def _get_preset_options_impl(config, substitutions, preset_name):
     for o in config.options(section_name):
         try:
             a = config.get(section_name, o)
-        except ConfigParser.InterpolationMissingOptionError, e:
+        except ConfigParser.InterpolationMissingOptionError as e:
             # e.reference contains the correctly formatted option
             missing_opts.append(e.reference)
             continue