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

Modules/main.c

@@ -799,7 +799,7 @@ pymain_run_python(PyInterpreterState *interp, int *exitcode)
         Py_DECREF(path0);
     }
 
-    PyCompilerFlags cf = {.cf_flags = 0};
+    PyCompilerFlags cf = {.cf_flags = 0, .cf_feature_version = PY_MINOR_VERSION};
 
     pymain_header(config);
     pymain_import_readline(config);