Cleanup code and pyproject (#608)

* use isort
* fallback: use BytesIO instead of StringIO. We had dropped Python 2
already.

test/test_except.py

@@ -1,10 +1,11 @@
 #!/usr/bin/env python
 
-from pytest import raises
-from msgpack import packb, unpackb, Unpacker, FormatError, StackError, OutOfData
-
 import datetime
 
+from pytest import raises
+
+from msgpack import FormatError, OutOfData, StackError, Unpacker, packb, unpackb
+
 
 class DummyException(Exception):
     pass