User can not cythonize during `pip install msgpack`.
So remove cython from build dependency.
If user need to use another Cython, user should download sdist, unzip,
manually cythonize, and `pip install .`.
The following steps have been taken:
1. Black was updated to latest version. The code has been formatted with
the new version.
2. The pyupgrade utility is installed. This helped to remove all the
code that was needed to support Python < 3.7.
Fix#541.
Co-authored-by: Inada Naoki <songofacandy@gmail.com>
This change causes Tox to run the project's setup.py in a virtualenv
(default path is .tox/.package). The required version of Cython is
installed, rather than whatever version is installed system wide.
