Heat oil in a Dutch oven over medium-high heat Add pumpkin (or squash) and cook, stirring occasionally, for 3 minutes. Add onion and cook, stirring frequently, for 4 minutes more. Add tomatoes, curry powder and water; bring to a boil. Reduce heat to low and simmer, uncovered and stirring occasionally, until the pumpkin or squash is tender but not mushy, 20 to 25 minutes. Season with salt and pepper and garnish with yogurt.