Ensure Python 3.7 in setup.py (#13347)

setup.py

@@ -5,8 +5,8 @@
 import os.path
 import sys
 
-if sys.version_info < (3, 6, 0):
-    sys.stderr.write("ERROR: You need Python 3.6 or later to use mypy.\n")
+if sys.version_info < (3, 7, 0):
+    sys.stderr.write("ERROR: You need Python 3.7 or later to use mypy.\n")
     exit(1)
 
 # we'll import stuff from the source tree, let's ensure is on the sys path
@@ -174,7 +174,6 @@ def run(self):
     "Intended Audience :: Developers",
     "License :: OSI Approved :: MIT License",
     "Programming Language :: Python :: 3",
-    "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",
@@ -219,7 +218,7 @@ def run(self):
         "python2": "typed_ast >= 1.4.0, < 2",
         "reports": "lxml",
     },
-    python_requires=">=3.6",
+    python_requires=">=3.7",
     include_package_data=True,
     project_urls={
         "News": "http://mypy-lang.org/news.html",