[2.7] bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192). (GH-8255)

(cherry picked from commit 504373c59b48f1ea12132d515459022730db6047)

Also backport tests for skipitem() and handling errors.
diff --git a/Python/getargs.c b/Python/getargs.c
index 32ff6fc..cc1ddde 100644
--- a/Python/getargs.c
+++ b/Python/getargs.c
@@ -1780,7 +1780,7 @@
                 else
                     (void) va_arg(*p_va, int *);
                 format++;
-            } else if ((c == 's' || c == 'z') && *format == '*') {
+            } else if ((c == 's' || c == 'z' || c == 'w') && *format == '*') {
                 format++;
             }
             break;