init: turn on demandimport for Python 3.6 and above

This uses the new demandimport implementation for Python 3 introduced in
previous patches.

This doesn't yet enhance performance because it isn't integrated with the
custom source file loader we use on Python 3. We'll integrate the two in
upcoming patches.

hg

@@ -28,8 +28,8 b" if libdir != '@' 'LIBDIR' '@':"
 
 # enable importing on demand to reduce startup time
 try:
-    if sys.version_info[0] < 3:
-        from mercurial import demandimport; demandimport.enable()
+    if sys.version_info[0] < 3 or sys.version_info >= (3, 6):
+        import hgdemandimport; hgdemandimport.enable()
 except ImportError:
     sys.stderr.write("abort: couldn't find mercurial libraries in [%s]\n" %
                      ' '.join(sys.path))