bpo-35975: Support parsing earlier minor versions of Python 3 (GH-12086)

This adds a `feature_version` flag to `ast.parse()` (documented) and `compile()` (hidden) that allow tweaking the parser to support older versions of the grammar. In particular if `feature_version` is 5 or 6, the hacks for the `async` and `await` keyword from PEP 492 are reinstated. (For 7 or higher, these are unconditionally treated as keywords, but they are still special tokens rather than `NAME` tokens that the parser driver recognizes.)



https://bugs.python.org/issue35975

Include/Python-ast.h

@@ -703,6 +703,7 @@ type_ignore_ty _Py_TypeIgnore(int lineno, PyArena *arena);
 
 PyObject* PyAST_mod2obj(mod_ty t);
 mod_ty PyAST_obj2mod(PyObject* ast, PyArena* arena, int mode);
+mod_ty PyAST_obj2mod_ex(PyObject* ast, PyArena* arena, int mode, int feature_version);
 int PyAST_Check(PyObject* obj);
 
 #ifdef __cplusplus