Adding HTU31D

Author: kattni

.gitmodules

@@ -781,3 +781,6 @@
 [submodule "libraries/helpers/datetime"]
 	path = libraries/helpers/datetime
 	url = https://github.com/adafruit/Adafruit_CircuitPython_datetime.git
+[submodule "libraries/drivers/htu31d"]
+	path = libraries/drivers/htu31d
+	url = https://github.com/adafruit/Adafruit_CircuitPython_HTU31D.git

docs/drivers.rst

@@ -349,6 +349,7 @@ equivalent carbon dioxide (``eco2`` / ``eCO2``), and total volatile organic comp
     DS18x20 Temperature <https://circuitpython.readthedocs.io/projects/ds18x20/en/latest/>
     HTS221 Temperature and Humidity Sensor <https://circuitpython.readthedocs.io/projects/hts221/en/latest/>
     HTU21D Temperature and Humidity <https://circuitpython.readthedocs.io/projects/htu21d/en/latest/>
+    HTU31D Temperature and Humidity <https://circuitpython.readthedocs.io/projects/htu31d/en/latest/>
     LPS2X Family of Barometric Pressure, Temperature Sensors <https://circuitpython.readthedocs.io/projects/lps2x/en/latest/>
     LPS35HW Water Resistant Barometric Pressure, Temperature <https://circuitpython.readthedocs.io/projects/lps35hw/en/latest/>
     SGP40 Air Quality Sensor <https://circuitpython.readthedocs.io/projects/sgp40/en/latest/>