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
diff --git a/Grammar/Tokens b/Grammar/Tokens
index 1d45e05..9de2da5 100644
--- a/Grammar/Tokens
+++ b/Grammar/Tokens
@@ -55,6 +55,8 @@
COLONEQUAL ':='
OP
+AWAIT
+ASYNC
TYPE_IGNORE
TYPE_COMMENT
ERRORTOKEN