Do not error highlight PEP 3017 operator

Fixes #11

syntax/python.vim

@@ -102,7 +102,7 @@ syn keyword pythonOperator      and in is not or
 if s:Enabled('g:python_highlight_operators')
     syn match pythonOperator        '\V=\|-\|+\|*\|@\|/\|%\|&\||\|^\|~\|<\|>\|!='
 endif
-syn match pythonError           '[$?]\|\([-+@%&|^~]\)\1\{1,}\|\([=*/<>]\)\2\{2,}\|\([-=+*@/%&|^~<>]\)\3\@![-+*@/%&|^~<>]\|[<!>]\+=\{2,}\|!\{2,}=\+' display
+syn match pythonError           '[$?]\|\([-+@%&|^~]\)\1\{1,}\|\([=*/<>]\)\2\{2,}\|\([+=*@/%&|^~<>]\)\3\@![-+*@/%&|^~<>]\|-[+*@/%&|^~<]\|[<!>]\+=\{2,}\|!\{2,}=\+' display
 
 "
 " Decorators (new in Python 2.4)

tests/test.py

@@ -117,6 +117,8 @@ and or in is not
 -= += *= **= @= /= //= %=
 &= |= ^= ~= <<= >>=
 
+->
+
 # Erroneous operators
 
 $ ?