Fix operator error highlighting for = prefix

Fixes #20

syntax/python.vim

@@ -103,7 +103,7 @@ 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

@@ -166,7 +166,7 @@ and or in is not
 & | ^ ~ << >>
 < <= == != >= >
 
-=
+= =- =+ =~
 -= += *= **= @= /= //= %=
 &= |= ^= ~= <<= >>=