johnwasser:
Is the sensor on a breakout board? Does the board contain anything other than a bypass capacitor and two pull-up resistors for the I2C bus?
Well, good question, what I have is this module:
I found it looking for SI7021, but now I am confused since printed on the board I find SI7021, GY-21, SHT21, and HTU21 
The SI7021 library gets temperature & humidity just fine though.
Here are the schematics of my circuit, I have an RTC module on the I2C bus along with the sensor:
Franck