TYP: define gen and gen2 as Iterable (#45139)

Co-authored-by: Kashif Khan <200~361477+kashifkhan@users.noreply.github.com>

Author: kashifkhan

pandas/io/formats/excel.py

@@ -649,20 +649,21 @@ def _format_header_regular(self) -> Iterable[ExcelCell]:
                 )
 
     def _format_header(self) -> Iterable[ExcelCell]:
+        gen: Iterable[ExcelCell]
+
         if isinstance(self.columns, MultiIndex):
             gen = self._format_header_mi()
         else:
             gen = self._format_header_regular()
 
-        gen2 = ()
+        gen2: Iterable[ExcelCell] = ()
+
         if self.df.index.names:
             row = [x if x is not None else "" for x in self.df.index.names] + [
                 ""
             ] * len(self.columns)
             if reduce(lambda x, y: x and y, map(lambda x: x != "", row)):
-                # error: Incompatible types in assignment (expression has type
-                # "Generator[ExcelCell, None, None]", variable has type "Tuple[]")
-                gen2 = (  # type: ignore[assignment]
+                gen2 = (
                     ExcelCell(self.rowcounter, colindex, val, self.header_style)
                     for colindex, val in enumerate(row)
                 )