UARTService.readinto with "memoryview" slice and "bytearrary" causes ValueError: length argument not allowed for this type

`byte_count = device.readinto(memoryview(in_buf)[start_ptr:])`

causes: `ValueError: length argument not allowed for this type`

also tried with "bytearray" and got same error.

```
Traceback (most recent call last):
File "", line 1, in
File "dev.py", line 308, in
File "lib/metre/gAsynco.py", line 121, in run
File "lib/metre/gAsynco.py", line 522, in run_until_complete
File "lib/metre/gAsynco.py", line 506, in run_until_complete
File "lib/metre/gAsynco.py", line 196, in _wrapped_coro
File "lib/metre/gAsynco.py", line 394, in _wrapper
File "dev.py", line 234, in serial_task
File "adafruit_ble/services/nordic.py", line 77, in readinto
ValueError: length argument not allowed for this type
```

I poked around and see this in nordic.py:

```
def readinto(self, buf, nbytes=None):
"""
Read bytes into the ``buf``. If ``nbytes`` is specified then read at most
that many bytes. Otherwise, read at most ``len(buf)`` bytes.
:return: number of bytes read and stored into ``buf``
:rtype: int or None (on a non-blocking error)
"""
return self._rx.readinto(buf, nbytes)
```

Looking futher in _blio.CharacteristicBuffer at suggestion from @dhalbert it looks like the `ValueError: length argument not allowed for this type is thrown` because two parameters are passed in `return self._rx.readinto(buf, nbytes)` above.

Start in adafruit_ble/services/nordic.py at UARTService.readinto, then inspect _blio.CharacteristicBuffer to confirm how its readinto arguments are handled. Reproduce the ValueError with a memoryview slice and bytearray, and consider the issue resolved when both buffer types work and the readinto return value remains correct.

Written by the indexing model from the issue text.