Is there any way to change the aspect ratio of the game so that it's not stretched to 16:9? I would prefer having a 4:3 aspect ratio with black bars. I'm using Windows 10 with a 1080p monitor.

If you are using a 4:3 resolution (ex: 1440x1080, 1280x960, 1024x768), then it is probably a problem with how the monitor is displaying the image. See this link for more information:

[url=https://pcgamingwiki.com/wiki/Glossary:Graphics_card#GPU_scaling]https://pcgamingwiki.com/wiki/Glossary:Graphics_card#GPU_scaling[/url]