Attempt to increase reliability

Reading from the UART can return bad values (this can also be tested by unplugging the cable or not fully fitting the cable).
With previous version of the library, at the first glitch, the library would never return a valid value.
This version agressively try to read a first byte until we have the start of frame signature or there is no more byte to read.
It give much better result for me, and this can be tested by playing with the cable.

Author: dglaude

adafruit_pm25.py

@@ -172,17 +172,17 @@ def __init__(self, uart, reset_pin=None):
         super().__init__()
 
     def _read_into_buffer(self):
-        b = self._uart.read(1)
-        if not b:
-            raise RuntimeError("Unable to read from PM2.5 UART")
-        b = b[0]
-        if b != 0x42:
-            raise RuntimeError("Unable to read from PM2.5 UART")
-        self._buffer[0] = b  # read one byte
+        while True:
+            b = self._uart.read(1)
+            if not b:
+                raise RuntimeError("Unable to read from PM2.5 (no start of frame)")
+            if b[0] == 0x42:
+                break
+        self._buffer[0] = b[0]  # first byte and start of frame
 
         remain = self._uart.read(31)
         if not remain or len(remain) != 31:
-            raise RuntimeError("Unable to read from PM2.5 UART")
+            raise RuntimeError("Unable to read from PM2.5 (incomplete frame)")
         for i in range(31):
             self._buffer[i + 1] = remain[i]
         # print([hex(i) for i in self._buffer])