Fixed string.Template syntax

python.vim

 
 if exists("python_highlight_string_templates") && python_highlight_string_templates != 0
   " String templates
-  syn match pythonStrTemplate	"\$\(\$\|{[^}]*}\|[a-zA-Z_][a-zA-Z0-9_]*\)" contained containedin=pythonString,pythonUniString,pythonRawString,pythonUniRawString
+  syn match pythonStrTemplate	"\$\(\$\|{[a-zA-Z_][a-zA-Z0-9_]*}\|[a-zA-Z_][a-zA-Z0-9_]*\)" contained containedin=pythonString,pythonUniString,pythonRawString,pythonUniRawString
 endif
 
 if exists("python_highlight_doctests") && python_highlight_doctests != 0