Drop Python 3.6 support (#543)

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>

test/test_limits.py

@@ -1,6 +1,4 @@
 #!/usr/bin/env python
-# coding: utf-8
-from __future__ import absolute_import, division, print_function, unicode_literals
 import pytest
 
 from msgpack import (