python: consider output header w/ expandable width

Problem: The expandable field prefix ("+:") does not consider
the width of the output header name when calculating output width.
If all output entries are less than the size of the output header
name, then output will not look as expected.

Solution: When calculating maximum output width of a field, take
into account the header entry name as long as no_header is not set.

Update some tests in python/t0024-util.py for changes.

Fixes #7030

Author: chu11

src/bindings/python/flux/util.py

@@ -1004,12 +1004,18 @@ def sentinel_keys():
 
                 #  Save the modified format, index, type, maximum width,
                 #  observed width, and broken-down spec in lst:
+                initialmaxwidth = spec.width or 0
+                if sentinels[end] in ("maxwidth", "both"):
+                    initialmaxwidth = max(
+                        initialmaxwidth,
+                        0 if no_header else len(self.headings.get(field, "")),
+                    )
                 lst.append(
                     dict(
                         fmt=fmt,
                         index=index,
                         type=sentinels[end],
-                        maxwidth=spec.width or 0,
+                        maxwidth=initialmaxwidth,
                         width=0,
                         spec=spec,
                     )

t/python/t0024-util.py

@@ -212,10 +212,11 @@ def test_filter(self):
         ).filter(items)
         self.assertEqual(fmt, "{i:>8} {f:8.2}")
 
+        # N.B. float has a width of 5 due to "FLOAT" header
         fmt = OutputFormat(
             "?+:{i:>7} ?:{s:>6} ?+:{f:.2f}", headings=self.headings
         ).filter(items)
-        self.assertEqual(fmt, "{i:>8} {f:3.2f}")
+        self.assertEqual(fmt, "{i:>8} {f:5.2f}")
 
     def test_sort(self):
         a = Item("a", 0, 2.2)
@@ -261,11 +262,13 @@ def test_issue6530(self):
         b = Item("abcdefghijklmnop", 2, 13)
         c = Item("c", 4, 5.0)
 
+        # N.B. iinteger has a width of 7 due to "INTEGER" header
+        # N.B. float has a width of 5 due to "FLOAT" header
         items = [a, b, c]
         fmt = OutputFormat(
             "+:{s:5.5} +:{i:4d} +:{f:.2f}", headings=self.headings
         ).filter(items)
-        self.assertEqual(fmt, "{s:16.16} {i:4d} {f:3.2f}")
+        self.assertEqual(fmt, "{s:16.16} {i:7d} {f:5.2f}")
 
     def test_copy(self):
         original = "+:{s:5.5} {i:4d} {f:.2f}"