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>

msgpack/ext.py

@@ -1,4 +1,3 @@
-# coding: utf-8
 from collections import namedtuple
 import datetime
 import sys
@@ -15,10 +14,10 @@ class ExtType(namedtuple("ExtType", "code data")):
             raise TypeError("data must be bytes")
         if not 0 <= code <= 127:
             raise ValueError("code must be 0~127")
-        return super(ExtType, cls).__new__(cls, code, data)
+        return super().__new__(cls, code, data)
 
 
-class Timestamp(object):
+class Timestamp:
     """Timestamp represents the Timestamp extension type in msgpack.
 
     When built with Cython, msgpack uses C methods to pack and unpack `Timestamp`. When using pure-Python
@@ -47,24 +46,18 @@ class Timestamp(object):
         if not isinstance(nanoseconds, int):
             raise TypeError("nanoseconds must be an integer")
         if not (0 <= nanoseconds < 10**9):
-            raise ValueError(
-                "nanoseconds must be a non-negative integer less than 999999999."
-            )
+            raise ValueError("nanoseconds must be a non-negative integer less than 999999999.")
         self.seconds = seconds
         self.nanoseconds = nanoseconds
 
     def __repr__(self):
         """String representation of Timestamp."""
-        return "Timestamp(seconds={0}, nanoseconds={1})".format(
-            self.seconds, self.nanoseconds
-        )
+        return f"Timestamp(seconds={self.seconds}, nanoseconds={self.nanoseconds})"
 
     def __eq__(self, other):
         """Check for equality with another Timestamp object"""
         if type(other) is self.__class__:
-            return (
-                self.seconds == other.seconds and self.nanoseconds == other.nanoseconds
-            )
+            return self.seconds == other.seconds and self.nanoseconds == other.nanoseconds
         return False
 
     def __ne__(self, other):
@@ -164,9 +157,7 @@ class Timestamp(object):
         :rtype: datetime.
         """
         utc = datetime.timezone.utc
-        return datetime.datetime.fromtimestamp(0, utc) + datetime.timedelta(
-            seconds=self.to_unix()
-        )
+        return datetime.datetime.fromtimestamp(0, utc) + datetime.timedelta(seconds=self.to_unix())
 
     @staticmethod
     def from_datetime(dt):