python-info-dedenter-opening-block-positions: Fix to support "bare" match

* lisp/progmodes/python.el
(python-info-dedenter-opening-block-positions): Make the check
stricter.  Require that block starts only at indentation.

* test/lisp/progmodes/python-tests.el
(python-indent-after-bare-match): Another test (bug#62031).

lisp/progmodes/python.el

@@ -5792,7 +5792,9 @@ likely an invalid python file."
             (while (python-nav--syntactically
                     (lambda ()
                       (cl-loop while (re-search-backward (python-rx block-start) nil t)
-                               if (memq (char-before) '(nil ?\s ?\t ?\n))
+                               if (save-match-data
+                                    (looking-back (rx line-start (* whitespace))
+                                                  (line-beginning-position)))
                                return t))
                     #'<)
               (let ((indentation (current-indentation)))