chore: Update version to 2.4.0 (#354)

Author: farleyb-amazon

src/aws_encryption_sdk/identifiers.py

@@ -27,7 +27,7 @@
     # We only actually need these imports when running the mypy checks
     pass
 
-__version__ = "2.3.0"
+__version__ = "2.4.0"
 USER_AGENT_SUFFIX = "AwsEncryptionSdkPython/{}".format(__version__)