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

Python/compile.c

@@ -330,6 +330,7 @@ PyAST_CompileObject(mod_ty mod, PyObject *filename, PyCompilerFlags *flags,
         goto finally;
     if (!flags) {
         local_flags.cf_flags = 0;
+        local_flags.cf_feature_version = PY_MINOR_VERSION;
         flags = &local_flags;
     }
     merged = c.c_future->ff_features | flags->cf_flags;