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/Include/Python-ast.h b/Include/Python-ast.h
index 0c739db..08d50ff 100644
--- a/Include/Python-ast.h
+++ b/Include/Python-ast.h
@@ -330,6 +330,7 @@
             expr_ty value;
             int conversion;
             expr_ty format_spec;
+            string expr_text;
         } FormattedValue;
 
         struct {
@@ -637,10 +638,10 @@
 expr_ty _Py_Call(expr_ty func, asdl_seq * args, asdl_seq * keywords, int
                  lineno, int col_offset, int end_lineno, int end_col_offset,
                  PyArena *arena);
-#define FormattedValue(a0, a1, a2, a3, a4, a5, a6, a7) _Py_FormattedValue(a0, a1, a2, a3, a4, a5, a6, a7)
+#define FormattedValue(a0, a1, a2, a3, a4, a5, a6, a7, a8) _Py_FormattedValue(a0, a1, a2, a3, a4, a5, a6, a7, a8)
 expr_ty _Py_FormattedValue(expr_ty value, int conversion, expr_ty format_spec,
-                           int lineno, int col_offset, int end_lineno, int
-                           end_col_offset, PyArena *arena);
+                           string expr_text, int lineno, int col_offset, int
+                           end_lineno, int end_col_offset, PyArena *arena);
 #define JoinedStr(a0, a1, a2, a3, a4, a5) _Py_JoinedStr(a0, a1, a2, a3, a4, a5)
 expr_ty _Py_JoinedStr(asdl_seq * values, int lineno, int col_offset, int
                       end_lineno, int end_col_offset, PyArena *arena);