kiss: Simplification: always prepend FEND

Since we empty the buffer after each frame, just blindly assume that we
need to add one.  Two adjacent `FEND`s is valid KISS protocol, and if
there was a bit of line noise, this will prevent that from getting mixed
up with our frame.

aioax25/kiss.py

@@ -330,10 +330,9 @@ def _pick_next_tx(self):
             # TX queue is empty
             return
 
-        if not self._tx_buffer.endswith(bytearray([BYTE_FEND])):
-            self._tx_buffer += bytearray([BYTE_FEND])
-
-        self._tx_buffer += bytes(rawframe) + bytearray([BYTE_FEND])
+        self._tx_buffer += (
+            bytearray([BYTE_FEND]) + bytes(rawframe) + bytearray([BYTE_FEND])
+        )
         self._tx_future = future
 
     def _receive_frame(self):