Simplify checking for 3to2

Switch to importing the lib3to2 as a check, which is platform agnostic
and doesn't depend on how 3to2 was installed. Also, take advantage of
the fact that the 3to2 converter recurses by default to avoid spawning
a bunch of subprocesses. Finally, don't depend on the location of the
3to2 script and just use the library directly to do the conversion.

interfaces/cython/SConscript

@@ -180,21 +180,20 @@ if localenv['python2_package'] == 'full':
 
     # Use 3to2 to convert examples from Python 3 syntax
     if env['python_convert_examples']:
-        def convert_example(target, source, env):
-            shutil.copyfile(source[0].abspath, target[0].abspath)
-            subprocess.call(env['threetotwo_cmd'] + ['--no-diff', '-n', '-w','-x', 'str',
-                         '-f', 'all', '-f', 'printfunction', '-x', 'print',
-                         '-x', 'open', target[0].abspath])
-        for subdir in subdirs('cantera/examples'):
-            dirpath = pjoin('cantera', 'examples', subdir)
-            for filename in os.listdir(dirpath):
-                if not filename.endswith('.py'):
-                    continue
-                targetdir = '../../build/python2/cantera/examples'
-                a = build(py2env.Command(pjoin(targetdir, subdir, filename),
-                                         pjoin(dirpath, filename),
-                                         convert_example))
-                py2env.Depends(mod, a)
+        a = build(py2env.Command('#build/python2/cantera/examples', 'cantera/examples',
+                                 Copy("$TARGET", "$SOURCE")))
+        from textwrap import dedent
+        # In these next few things, the double quotes are necessary
+        # so the command line is processed properly
+        threetotwo_options = ['--no-diff', '-n', '-w', '-x', 'str', '-x', 'print', '-x', 'open',
+                              '-f', 'all', '-f', 'printfunction']
+        threetotwo_options = ', '.join(["'{}'"]*len(threetotwo_options)).format(*threetotwo_options)
+        ex_loc = "'build/python2/cantera/examples'"
+        convert_script = dedent("""\
+        $python2_cmd_esc -c "from lib3to2.main import main; main('lib3to2.fixes', [{opts}, {loc}])"
+        """.format(opts=threetotwo_options, loc=ex_loc))
+        b = build(py2env.AddPostAction(a, convert_script))
+        py2env.Depends(mod, b)
 
     add_dependencies(mod, ext)
     install_module(py2env['python2_prefix'], py2_version)