Add walrus operator

Closes #77

syntax/python.vim

@@ -114,7 +114,7 @@ endif
 "
 syn keyword pythonOperator      and in is not or
 if s:Enabled('g:python_highlight_operators')
-    syn match pythonOperator        '\V=\|-\|+\|*\|@\|/\|%\|&\||\|^\|~\|<\|>\|!='
+    syn match pythonOperator        '\V=\|-\|+\|*\|@\|/\|%\|&\||\|^\|~\|<\|>\|!=\|:='
 endif
 syn match pythonError           '[$?]\|\([-+@%&|^~]\)\1\{1,}\|\([=*/<>]\)\2\{2,}\|\([+@/%&|^~<>]\)\3\@![-+*@/%&|^~<>]\|\*\*[*@/%&|^<>]\|=[*@/%&|^<>]\|-[+*@/%&|^~<]\|[<!>]\+=\{2,}\|!\{2,}=\+' display