add AHT20 sensor

Author: shaoziyang

sensor/AHT20/READMD.md

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+# [mpy-lib](https://github.com/micropython-Chinese-Community/mpy-lib)
+
+## AHT20 SENSOR 
+
+
+## Example
+
+
+```py
+from machine import I2C, Pin
+from aht20 import AHT20
+from time import sleep_ms
+
+i2c = I2C(0, sda = Pin(4), scl = Pin(5))
+aht = AHT20(i2c)
+
+while 1:
+    aht.measure()
+    print('Humi: {:.2f} %, Temp: {:.2f} C'.format(aht.Humi(), aht.Temp()))
+    sleep_ms(1000)
+```

sensor/AHT20/aht20.py

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+'''
+    mpy drive for AHT20 Sensor
+
+    Author: shaoziyang
+    Date:   2023.8
+
+    http://www.micropython.org.cn
+
+'''
+from machine import I2C
+from micropython import const
+from time import sleep_ms
+
+CMD_INIT = const(0xBE)
+CMD_MEASURE = const(0xAC)
+CMD_RESET = const(0xBA)
+
+class AHT20:
+
+    def __init__(self, i2c):
+        self.i2c = i2c
+        self.addr = 56
+        self.tb = bytearray(3)
+        self.rb = bytearray(6)
+        self._H = 0
+        self._T = 0
+        self.init()
+
+    def set(self, cmd, dat):
+        self.tb[0] = cmd
+        self.tb[1] = dat >> 8
+        self.tb[2] = dat
+        self.i2c.writeto_mem(self.addr, cmd, self.tb)
+
+    def get(self):
+        self.i2c.readfrom_into(self.addr, self.rb)
+
+    def init(self):
+        self.set(CMD_INIT, 0x0800)
+
+    def reset(self):
+        self.set(CMD_RESET, 0)
+
+    def measure(self):
+        self.set(CMD_MEASURE, 0x3300)
+        sleep_ms(75)
+        self.get()
+        self._H = (self.rb[1]<<12)+(self.rb[2]<<4)+(self.rb[3]>>4)
+        self._T = ((self.rb[3]%16)<<16)+(self.rb[4]<<8)+self.rb[5]
+
+    def Humi(self):
+        return (self._H*100)/(1<<20)
+
+    def Temp(self):
+        return (self._T*200)/(1<<20)-50
+
+