Solution

Assuming we're working with ideal gases
For an ideal gas, the internal energy only depends on the temperature
$U=nC_v T$ , where n is the number of moles and C_v is the molar heat capacity at constant volume
Careful here, we could think that obviously if the temperature increases, the internal energy increases too, let's think this
We have
$PV=nRT=const$, because P and V are both constant
but this is only possible if n changed (air can escape), because if n is constant, the product nRT would vary with T, contradicting that PV is constant
So, from $nRT=const$ , we get
$n_0 T_0=nT$
and substituting this into the expression for the final energy
$U=nC_v T$
$U=n_0 C_v T_0$ , which is exactly the initial energy U_0
so $U_0=U$
the internal energy did not change