support for async keyword

syntax/python.vim

   syn match   pythonStatement   "\<yield\s\+from\>" display
   syn keyword pythonBuiltinObj  None True False
   syn match   pythonFunction    "\%([^[:cntrl:][:space:][:punct:][:digit:]]\|_\)\%([^[:cntrl:][:punct:][:space:]]\|_\)*" display contained
-  syn keyword pythonStatement   await
+  syn keyword pythonStatement   await async
   syn match   pythonStatement   "\<async\s\+def\>" nextgroup=pythonFunction skipwhite
   syn match   pythonStatement   "\<async\s\+with\>" display
   syn match   pythonStatement   "\<async\s\+for\>" display