Add some types to _mathtext.py

Mypy 1.16.0 flags errors here:

lib/matplotlib/_mathtext.py:2531: error: "Node" has no attribute "width"  [attr-defined]
lib/matplotlib/_mathtext.py:2608: error: List item 0 has incompatible type "Kern"; expected "Hlist | Vlist"  [list-item]

The check for the attribute _metrics is equivalent to checking for an
instance of Char, since only Char and its subclasses set self._metrics.

Mypy infers an unnecessarily tight type list[Hlist | Vlist] for
spaced_nucleus so we give it a more general one.

Author: jkseppan

lib/matplotlib/_mathtext.py

@@ -2524,10 +2524,10 @@ def subsuper(self, s: str, loc: int, toks: ParseResults) -> T.Any:
             if len(new_children):
                 # remove last kern
                 if (isinstance(new_children[-1], Kern) and
-                        hasattr(new_children[-2], '_metrics')):
+                        isinstance(new_children[-2], Char)):
                     new_children = new_children[:-1]
                 last_char = new_children[-1]
-                if hasattr(last_char, '_metrics'):
+                if isinstance(last_char, Char):
                     last_char.width = last_char._metrics.advance
             # create new Hlist without kerning
             nucleus = Hlist(new_children, do_kern=False)
@@ -2603,7 +2603,7 @@ def subsuper(self, s: str, loc: int, toks: ParseResults) -> T.Any:
 
         # Do we need to add a space after the nucleus?
         # To find out, check the flag set by operatorname
-        spaced_nucleus = [nucleus, x]
+        spaced_nucleus: list[Node] = [nucleus, x]
         if self._in_subscript_or_superscript:
             spaced_nucleus += [self._make_space(self._space_widths[r'\,'])]
             self._in_subscript_or_superscript = False