py/objstr: Support '{:08}'.format("Jan") like Python 3.10.

The new test has an .exp file, because it is not compatible with Python 3.9
and lower.

See CPython version of the issue at https://bugs.python.org/issue27772

Signed-off-by: Jeff Epler <jepler@gmail.com>

py/objstr.c

@@ -1163,7 +1163,7 @@ STATIC vstr_t mp_obj_str_format_helper(const char *str, const char *top, int *ar
                 s++;
             }
             if (*s == '0') {
-                if (!align) {
+                if (!align && arg_looks_numeric(arg)) {
                     align = '=';
                 }
                 if (!fill) {

tests/basics/string_format_cp310.py

@@ -0,0 +1,9 @@
+# Python 3.10+ functionality test for {} format string
+
+def test(fmt, *args):
+    print('{:8s}'.format(fmt) + '>' +  fmt.format(*args) + '<')
+
+test("{:0s}", "ab")
+test("{:06s}", "ab")
+test("{:<06s}", "ab")
+test("{:>06s}", "ab")

tests/basics/string_format_cp310.py.exp

@@ -0,0 +1,4 @@
+{:0s}   >ab<
+{:06s}  >ab0000<
+{:<06s} >ab0000<
+{:>06s} >0000ab<

tests/basics/string_format_error.py

@@ -1,7 +1,7 @@
 # tests for errors in {} format string
 
 try:
-    '{0:0}'.format('zzz')
+    '{0:=}'.format('zzz')
 except (ValueError):
     print('ValueError')