bpo-36817: Add f-string debugging using '='. (GH-13123)

If a "=" is specified a the end of an f-string expression, the f-string will evaluate to the text of the expression, followed by '=', followed by the repr of the value of the expression.
diff --git a/Parser/Python.asdl b/Parser/Python.asdl
index 668d3c9..626fa4f 100644
--- a/Parser/Python.asdl
+++ b/Parser/Python.asdl
@@ -76,7 +76,7 @@
          -- x < 4 < 3 and (x < 4) < 3
          | Compare(expr left, cmpop* ops, expr* comparators)
          | Call(expr func, expr* args, keyword* keywords)
-         | FormattedValue(expr value, int? conversion, expr? format_spec)
+         | FormattedValue(expr value, int? conversion, expr? format_spec, string? expr_text)
          | JoinedStr(expr* values)
          | Constant(constant value, string? kind)