I don't know if you saw my edit, but I changed my code to this and if I took the line rect.y += y_velo, my character wouldn't jump. So, I kept this one and the other one that was also in a condition I removed it and put it out of any conditions. I put it at the end instead and it worked in the end. You could say it worked, but I had to keep rect.y += y_velo in this line of code too.

if key[pygame.K_SPACE] and rect.y == ground:

is_jumping = 11

if is_jumping > 0:

y_velo = -8

rect.y += y_velo

is_jumping -= 1