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>

setup.py

@@ -1,5 +1,4 @@
 #!/usr/bin/env python
-# coding: utf-8
 import io
 import os
 import sys
@@ -25,7 +24,7 @@ except ImportError:
 
 
 def cythonize(src):
-    sys.stderr.write("cythonize: %r\n" % (src,))
+    sys.stderr.write(f"cythonize: {src!r}\n")
     cython_compiler.compile([src], cplus=True)
 
 
@@ -36,11 +35,7 @@ def ensure_source(src):
         if not have_cython:
             raise NoCython
         cythonize(pyx)
-    elif (
-        os.path.exists(pyx)
-        and os.stat(src).st_mtime < os.stat(pyx).st_mtime
-        and have_cython
-    ):
+    elif os.path.exists(pyx) and os.stat(src).st_mtime < os.stat(pyx).st_mtime and have_cython:
         cythonize(pyx)
     return src