Python: Don't highlight sub-identifiers as keywords

For example, highlight `file` as a builtin when appearing by itself,
but not in a construct like `obj.file`.

Fixes #1542.

data/filedefs/filetypes.python

@@ -34,6 +34,7 @@ identifiers=ArithmeticError AssertionError AttributeError BaseException Blocking
 [lexer_properties]
 fold.comment.python=1
 fold.quotes.python=1
+lexer.python.keywords2.no.sub.identifiers=1
 
 [settings]
 # default extension used when saving files