bug#20860: python.el's fill-paragraph incorrectly handles doc-strings with long first lines

[Deneb Meketa <deneb@pixar.com>]

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Attached please find a patch for consideration.

I also ran into this issue, and found this fix.  When we narrow to perform
the main fill, we need to preserve the entire first line.  Otherwise the
fill operation won't see the beginning of the first line; it will think
that line begins at the opening quote, and thus will allow the first line
to be longer than it should.

I have tested this in 26.1 with docstrings and ordinary strings, single and
triple quotes, auto-fill and fill-paragraph, docstring styles pep-257 and
django.  As far as I can tell, this function has not changed since 26.1.

Proposed commit message:

Python: fix overlong first line in string fill.

When we narrow to perform filling, make sure to include the entire first
line in
the narrowed view.  Otherwise the fill doesn't work correctly.

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--- python.OLD.el	2021-04-19 16:18:39.840528000 -0700
+++ python.NEW.el	2021-04-19 16:22:37.874439000 -0700
@@ -3945,16 +3945,21 @@ (defun python-fill-string (&optional jus
           (set-marker
            (make-marker)
            (or (python-syntax-context 'string)
                (and (equal (string-to-syntax "|")
                            (syntax-after (point)))
                     (point)))))
          (num-quotes (python-syntax-count-quotes
                       (char-after str-start-pos) str-start-pos))
+         (str-line-start-pos
+          (save-excursion
+            (goto-char str-start-pos)
+            (beginning-of-line)
+            (point-marker)))
          (str-end-pos
           (save-excursion
             (goto-char (+ str-start-pos num-quotes))
             (or (re-search-forward (rx (syntax string-delimiter)) nil t)
                 (goto-char (point-max)))
             (point-marker)))
          (multi-line-p
           ;; Docstring styles may vary for oneliners and multi-liners.
@@ -3968,17 +3973,17 @@ (defun python-fill-string (&optional jus
             ;; graphic idea of each style.
             (`django (cons 1 1))
             (`onetwo (and multi-line-p (cons 1 2)))
             (`pep-257 (and multi-line-p (cons nil 2)))
             (`pep-257-nn (and multi-line-p (cons nil 1)))
             (`symmetric (and multi-line-p (cons 1 1)))))
          (fill-paragraph-function))
     (save-restriction
-      (narrow-to-region str-start-pos str-end-pos)
+      (narrow-to-region str-line-start-pos str-end-pos)
       (fill-paragraph justify))
     (save-excursion
       (when (and (python-info-docstring-p) python-fill-docstring-style)
         ;; Add the number of newlines indicated by the selected style
         ;; at the start of the docstring.
         (goto-char (+ str-start-pos num-quotes))
         (delete-region (point) (progn
                                  (skip-syntax-forward "> ")