fix: type error when exporting file as bytes
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
![image](https://github.com/user-attachments/assets/50cb4a49-df23-4673-8d76-9db5dca7cc0a)
[official doc url](https://pandas.pydata.org/pandas-docs/stable/user_guide/io.html#writing-excel-files)
"Writing Excel files to memory" part


Thanks for the PR! It seems to me that `WriteExcelBuffer` was should have `io.BytesIO` as a subtype, is this not the case? No, while it seems to be a subtype, it doesn't actually work.
before
![image](https://github.com/user-attachments/assets/d1b3f11a-0545-4714-a519-c02e2d3e4d76)
![image](https://github.com/user-attachments/assets/9e30736f-6338-486f-b97d-1c29bd5b3e22)
I tried. It's the best solution I've got. line 1242
after
![image](https://github.com/user-attachments/assets/5886ce2c-898a-4e23-9e1c-3d4c7b87d4cf)
![image](https://github.com/user-attachments/assets/0d08fb86-c807-4fbf-a6a9-604491395b77)

 > No, while it seems to be a subtype, it doesn't actually work.

I'm against adding unnecessary type-hints to make IDEs happy. If it is a subtype, then the IDE should be fine with it and this is an issue with the IDE, not pandas. Okay, I'll double-check if it's an IDE problem.