update CHANGELOG and bump version to 5.3.1

===== New Features

* Added support for shortening dicts in local variables {pull}638[#638]

Author: basepi

elasticapm/version.py

@@ -28,5 +28,5 @@
 #  OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 #  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-__version__ = (5, 3, 0)
+__version__ = (5, 3, 1)
 VERSION = ".".join(map(str, __version__))