Drop python 3.8 (CrayLabs#544)

Python 3.8 is nearing its end of life so we're no longer supporting it.
[ committed by @AlyssaCote ]
[ reviewed by @MattToast @mellis13 ]

doc/changelog.rst

@@ -19,6 +19,7 @@ To be released at some future point in time
 Description
 
 - Fix race condition for telemetry monitor
+- Drop Python 3.8 support
 - Update watchdog dependency
 - Historical output files stored under .smartsim directory
 - Add option to build Torch backend without the Intel Math Kernel Library
@@ -41,6 +42,8 @@ Description
 Detailed Notes
 
 - Ensure the telemetry monitor is started prior to launching entities (SmartSim-PR549_)
+- Python 3.8 is reaching its end-of-life in October, 2024, so it will
+  no longer continue to be supported. (SmartSim-PR544_) 
 - Update watchdog dependency from 3.x to 4.x, fix new type issues (SmartSim-PR540_)
 - The dashboard needs to display historical logs, so log files are written
   out under the .smartsim directory and files under the experiment
@@ -98,6 +101,7 @@ Detailed Notes
   the previously registered signal handler. (SmartSim-PR535_)
 
 .. _SmartSim-PR549: https://github.com/CrayLabs/SmartSim/pull/549
+.. _SmartSim-PR544: https://github.com/CrayLabs/SmartSim/pull/544
 .. _SmartSim-PR540: https://github.com/CrayLabs/SmartSim/pull/540
 .. _SmartSim-PR532: https://github.com/CrayLabs/SmartSim/pull/532
 .. _SmartSim-PR538: https://github.com/CrayLabs/SmartSim/pull/538