将PWM.duty替换成了PWM.duty_u16 (#2)

* Update st7735_buf.py

In pi pico , PWM.duty is not defined rather PWM.duty_u16
Thus if you use st7735 driver there will throw a error that
  AttributeError: type object 'PWM' has no attribute 'duty'
fixed duty(1023)  to  duty_u16(1023)

* Update st7735_spi.py

* Update st7789_buf.py

* Update st7789_spi.py

Author: XusMochizuki

driver/st7735_buf.py

@@ -144,7 +144,7 @@ def __init__(self, width: int, height: int, spi, res: int, dc: int,
             self.cs = Pin(cs, Pin.OUT, Pin.PULL_DOWN)
         if bl is not None:
             self.bl = PWM(Pin(bl, Pin.OUT))
-            self.bl.duty(1023)
+            self.bl.duty_u16(1023)
         else:
             self.bl = None
         self._rotate = rotate

driver/st7735_spi.py

@@ -142,7 +142,7 @@ def __init__(self, width: int, height: int, spi, res: int, dc: int,
             self.cs = Pin(cs, Pin.OUT, Pin.PULL_DOWN)
         if bl is not None:
             self.bl = PWM(Pin(bl, Pin.OUT))
-            self.bl.duty(1023)
+            self.bl.duty_u16(1023)
         else:
             self.bl = None
         self._rotate = rotate

driver/st7789_buf.py

@@ -179,7 +179,7 @@ def __init__(self, width: int, height: int, spi, res: int, dc: int,
             self.cs = Pin(cs, Pin.OUT, Pin.PULL_DOWN)
         if bl is not None:
             self.bl = PWM(Pin(bl, Pin.OUT))
-            self.bl.duty(1023)
+            self.bl.duty_u16(1023)
         else:
             self.bl = None
         self._rotate = rotate

driver/st7789_spi.py

@@ -177,7 +177,7 @@ def __init__(self, width: int, height: int, spi, res: int, dc: int,
             self.cs = Pin(cs, Pin.OUT, Pin.PULL_DOWN)
         if bl is not None:
             self.bl = PWM(Pin(bl, Pin.OUT))
-            self.bl.duty(1023)
+            self.bl.duty_u16(1023)
         else:
             self.bl = None
         self._rotate = rotate