[ty] Don't suggest keyword statements when only expressions are valid

There are cases where the python grammar enforces expressions
after certain statements. In such cases we want to suppress
irrelevant keywords from the auto-complete suggestions.

E.g. `with a<CURSOR>`, suggesting `raise` here never makes sense
because it is not valid by the grammar.

Author: RasmusNygren

crates/ty_completion_eval/completion-evaluation-tasks.csv

@@ -15,7 +15,7 @@ import-deprioritizes-type_check_only,main.py,3,2
 import-deprioritizes-type_check_only,main.py,4,3
 import-keyword-completion,main.py,0,1
 internal-typeshed-hidden,main.py,0,2
-none-completion,main.py,0,2
+none-completion,main.py,0,1
 numpy-array,main.py,0,159
 numpy-array,main.py,1,1
 object-attr-instance-methods,main.py,0,1