Using a voltage divider versus step-down voltage regulator for i2c pull-up
The best option, when possible, especially in the case where there is a single I2C master, is to pull up to the same VCC as the I2C master. In your application that would be the Raspberry Pi VCC, I guess.
But it is still an interesting question. The basic answer is, yes, you can use a divider for pullup.
The effective pullup voltage is V * R2 / (R1+R2)  where R1 is the upper leg of the divider and R2 is the lower leg of the divider. The effective pullup resistance is R1 in parallel with R2, which is (R1*R2)/(R1+R2).



simulate this circuit &ndash; Schematic created using CircuitLab
Note that the divider will consume power even when SDA and SCL are not pulled down. In micropower designs, the divider approach may not be the wisest option for that reason. An option (a bit odd, but potentially workable) would be to use a low quiescent current (Iq) linear regulator (possibly an LDO) to generate the 3.3V for the pullups. I have never seen this or had to resort to it, so I suspect it is not commonly needed.