bpo-39520: Fix un-parsing of ext slices with no dimensions (GH-18304)

(cherry picked from commit 185903de12)

Co-authored-by: Batuhan Taşkaya <47358913+isidentical@users.noreply.github.com>

Lib/test/test_future.py

@@ -256,6 +256,11 @@ class AnnotationsFutureTestCase(unittest.TestCase):
         eq("slice[:-1]")
         eq("slice[1:]")
         eq("slice[::-1]")
+        eq("slice[:,]")
+        eq("slice[1:2,]")
+        eq("slice[1:2:3,]")
+        eq("slice[1:2, 1]")
+        eq("slice[1:2, 2, 3]")
         eq("slice[()]")
         eq("slice[a, b:c, d:e:f]")
         eq("slice[(x for x in a)]")

Misc/NEWS.d/next/Core and Builtins/2020-02-02-00-12-07.bpo-39520.uicBq6.rst

@@ -0,0 +1,2 @@
+Fix unparsing of ext slices with no items (``foo[:,]``). Patch by Batuhan
+Taskaya.

Python/ast_unparse.c

@@ -746,6 +746,7 @@ append_ast_ext_slice(_PyUnicodeWriter *writer, slice_ty slice)
         APPEND_STR_IF(i > 0, ", ");
         APPEND(slice, (slice_ty)asdl_seq_GET(slice->v.ExtSlice.dims, i));
     }
+    APPEND_STR_IF(dims_count == 1, ",");
     return 0;
 }