Remove outdated Python pre-3.7 references (#1186)

Author: bschoening

README-dev.rst

@@ -176,7 +176,7 @@ Use tee to capture logs and see them on your terminal::
 
 Testing Multiple Python Versions
 --------------------------------
-If you want to test all of python 2.7, 3.5, 3.6, 3.7, and pypy, use tox (this is what
+If you want to test all of python 3.7, 3.8, and pypy, use tox (this is what
 TravisCI runs)::
 
     tox

test-requirements.txt

@@ -6,13 +6,10 @@ ccm>=2.1.2
 pytz
 sure
 pure-sasl
-twisted[tls]; python_version >= '3.5'
-twisted[tls]==19.2.1; python_version < '3.5'
+twisted[tls]
 gevent>=1.0
 eventlet
 cython>=0.20,<0.30
 packaging
-backports.ssl_match_hostname; python_version < '2.7.9'
 futurist; python_version >= '3.7'
-asynctest; python_version >= '3.5'
-ipaddress; python_version < '3.3.0'
+asynctest

tests/integration/long/test_ssl.py

@@ -28,7 +28,7 @@
 
 if not hasattr(ssl, 'match_hostname'):
     try:
-        from backports.ssl_match_hostname import match_hostname
+        from ssl import match_hostname
         ssl.match_hostname = match_hostname
     except ImportError:
         pass  # tests will fail