Remove Python 2.7 support. (#5425)

* remove 2.7 from CI and publishing

* fill out classifiers and also make it not be installed on 3.5

* some minor bumps so that the old deps work on python 3.5

synapse/__init__.py

@@ -17,6 +17,13 @@
 """ This is a reference implementation of a Matrix home server.
 """
 
+import sys
+
+# Check that we're not running on an unsupported Python version.
+if sys.version_info < (3, 5):
+    print("Synapse requires Python 3.5 or above.")
+    sys.exit(1)
+
 try:
     from twisted.internet import protocol
     from twisted.internet.protocol import Factory