I just noticed that micropython apparently doesn't implement the count() method for class bytearray().
Since it's a basic built-in class/type and I didn't find it anywhere mentioned (docs/issues), I figured it might be worth dropping it here.

CPython:

bytearray(b'foobar').count(b'o')
2

MicroPython:

>>> bytearray(b'foobar').count(b'o')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'bytearray' object has no attribute 'count'