Fix mypy issues following the update to mypy 0.740

>>> Checking type annotations
nxdrive/client/remote_client.py:336: error: No overload variant of "write" of "BinaryIO" matches argument type "memoryview"
nxdrive/client/remote_client.py:336: note: Possible overload variants:
nxdrive/client/remote_client.py:336: note:     def write(self, s: bytearray) -> int
nxdrive/client/remote_client.py:336: note:     def write(self, s: bytes) -> int
nxdrive/manager.py:636: error: Incompatible return value type (got "Optional[str]", expected "str")
nxdrive/commandline.py:574: error: On Python 3 '{}'.format(b'abc') produces "b'abc'"; use !r if this is a desired behavior