Require Python 3.7 for start_tls support.

Support start_tls is required to allow proxying HTTPS connections,
since the unencrypted HTTP connection gets upgraded to HTTPS in-place
right after the CONNECT succeeds.

setup.py

@@ -21,8 +21,8 @@
 
 exec((root_dir / 'src' / 'websockets' / 'version.py').read_text(encoding='utf-8'))
 
-if sys.version_info[:3] < (3, 6, 1):
-    raise Exception("websockets requires Python >= 3.6.1.")
+if sys.version_info[:3] < (3, 7):
+    raise Exception("websockets requires Python >= 3.7.")
 
 packages = ['websockets', 'websockets/extensions']
 
@@ -51,7 +51,6 @@
         'Operating System :: OS Independent',
         'Programming Language :: Python',
         'Programming Language :: Python :: 3',
-        'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
     ],
@@ -61,6 +60,6 @@
     ext_modules=ext_modules,
     include_package_data=True,
     zip_safe=False,
-    python_requires='>=3.6.1',
+    python_requires='>=3.7',
     test_loader='unittest:TestLoader',
 )