Cleanup code and pyproject (#608)

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

setup.py

@@ -1,11 +1,11 @@
 #!/usr/bin/env python
 import os
 import sys
-from setuptools import setup, Extension
+
+from setuptools import Extension, setup
 from setuptools.command.build_ext import build_ext
 from setuptools.command.sdist import sdist
 
-
 PYPY = hasattr(sys, "pypy_version_info")