chore: Update changelog and readme for 2.4.0 (#353)

Author: farleyb-amazon

CHANGELOG.rst

@@ -2,6 +2,19 @@
 Changelog
 *********
 
+2.4.0 -- 2021-07-01
+===================
+
+Deprecation Announcement
+------------------------
+The AWS Encryption SDK for Python is discontinuing support for Python 2. Future major versions of this library
+will drop support for Python 2 and begin to adopt changes that are known to break Python 2.
+
+Support for Python 3.4 will be removed at the same time. Moving forward, we will support Python 3.5+.
+
+Security updates will still be available for the Encryption SDK 2.x line for the next 12 months, in accordance with our `Support Policy <https://github.com/aws/aws-encryption-sdk-python/blob/master/SUPPORT_POLICY.rst>`__.
+
+
 2.3.0 -- 2021-06-16
 ===================
 

README.rst

@@ -35,6 +35,11 @@ Required Prerequisites
 ======================
 
 * Python 2.7+ or 3.4+
+
+  **NOTE: 2.x is the last major version of this library that will
+  support Python 2. Future major versions will begin to adopt changes
+  known to break Python 2. Python 3.4 support will also be removed
+  in future major versions; Python 3.5+ will be required.**
 * cryptography >= 2.5.0
 * boto3 >= 1.10.0
 * attrs