In a blog post from a few years ago, [Peter Molnar] explains in detail how to wire up a physical adapter to add a USB-connected I2C interface to a system. At its core is the ATtiny85 AVR-based MCU ...

We used the Bus Pirate universal serial interface tool to work with the ... but it would also work fine at 3.3volts. The I2C bus requires pull-up resistors on both bus wires.