Force removal of the old build dir in setup.py

Ideally this wouldn't be necessary, but it avoids the nasty scenario
of breaking a user's Py3 installation if the user happens to run:

    python2 setup.py build
    python3 setup.py install

We may be able to get around this by using the package_dir option in
setuptools.setup()

Author: edschofield

setup.py

@@ -83,7 +83,9 @@
                      'discover_tests.py',
                      'check_rst.sh',
                      'TESTING.txt',
-                    ]}
+                    ],
+                }
+
 REQUIRES = []
 TEST_REQUIRES = []
 if sys.version_info[:2] == (2, 6):
@@ -113,6 +115,19 @@
 setup_kwds = {}
 
 
+# * Important *
+# We forcibly remove the build folder to avoid breaking the
+# user's Py3 installation if they run "python2 setup.py
+# build" and then "python3 setup.py install".
+
+import distutils.dir_util
+print('removing old build dir')
+try:
+    distutils.dir_util.remove_tree('build')
+except OSError:
+    pass
+
+
 setup(name=NAME,
       version=VERSION,
       author=AUTHOR,