If Earth's orbit were a perfect circle about the sun so that the distance would never change, how would that effect the season

The annual seasons would always be the same over the lifetime of the earth.

Explanation:

However due to the elliptical nature of the earth's orbit sometime the earth is closer to the sun (aphelion) while in some occasions the planet is farthest from the sun (perihelion).

During aphelion, the general climate of the earth, hence the seasons, is warmer while during perihelion, the general climate of the earth is cooler. In addition due to the slower orbital velocity of earth during perihelion there is a longer time in change between seasons than in aphelion.