Bump version for continued development towards 5.4.0 (#354)

This PR updates the `setup.py` version for continued development towards
version 5.4.0.

Author: mdickinson

setup.py

@@ -19,10 +19,10 @@
 # together with information from the version control system, and then injected
 # into the package source.
 MAJOR = 5
-MINOR = 3
+MINOR = 4
 MICRO = 0
 PRERELEASE = ""
-IS_RELEASED = True
+IS_RELEASED = False
 
 # If this file is part of a Git export (for example created with "git archive",
 # or downloaded from GitHub), ARCHIVE_COMMIT_HASH gives the full hash of the