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_stricttype.py

@@ -1,5 +1,3 @@
-# coding: utf-8
-
 from collections import namedtuple
 from msgpack import packb, unpackb, ExtType
 
@@ -10,7 +8,7 @@ def test_namedtuple():
     def default(o):
         if isinstance(o, T):
             return dict(o._asdict())
-        raise TypeError("Unsupported type %s" % (type(o),))
+        raise TypeError(f"Unsupported type {type(o)}")
 
     packed = packb(T(1, 42), strict_types=True, use_bin_type=True, default=default)
     unpacked = unpackb(packed, raw=False)
@@ -23,7 +21,7 @@ def test_tuple():
     def default(o):
         if isinstance(o, tuple):
             return {"__type__": "tuple", "value": list(o)}
-        raise TypeError("Unsupported type %s" % (type(o),))
+        raise TypeError(f"Unsupported type {type(o)}")
 
     def convert(o):
         if o.get("__type__") == "tuple":
@@ -44,9 +42,7 @@ def test_tuple_ext():
     def default(o):
         if isinstance(o, tuple):
             # Convert to list and pack
-            payload = packb(
-                list(o), strict_types=True, use_bin_type=True, default=default
-            )
+            payload = packb(list(o), strict_types=True, use_bin_type=True, default=default)
             return ExtType(MSGPACK_EXT_TYPE_TUPLE, payload)
         raise TypeError(repr(o))
 
@@ -54,7 +50,7 @@ def test_tuple_ext():
         if code == MSGPACK_EXT_TYPE_TUPLE:
             # Unpack and convert to tuple
             return tuple(unpackb(payload, raw=False, ext_hook=convert))
-        raise ValueError("Unknown Ext code {}".format(code))
+        raise ValueError(f"Unknown Ext code {code}")
 
     data = packb(t, strict_types=True, use_bin_type=True, default=default)
     expected = unpackb(data, raw=False, ext_hook=convert)