parser: STType.compile() can be called with no arguments. (#4034)

The filename parameter is optional:
https://docs.python.org/3/library/parser.html#parser.ST.compile.

stdlib/2and3/parser.pyi

@@ -22,7 +22,7 @@ def issuite(st: STType) -> bool: ...
 class ParserError(Exception): ...
 
 class STType:
-    def compile(self, filename: _Path) -> CodeType: ...
+    def compile(self, filename: _Path = ...) -> CodeType: ...
     def isexpr(self) -> bool: ...
     def issuite(self) -> bool: ...
     def tolist(self, line_info: bool = ..., col_info: bool = ...) -> List[Any]: ...