remove pkg_resources warning

run_sim.py

 from ObservationSim.ObservationSim import Observation
 from ObservationSim._util import parse_args, make_run_dirs, generate_pointing_list
-from pkg_resources import get_distribution
+from importlib.metadata import version
 import os
 import yaml
 import shutil
@@ -24,7 +24,7 @@ def run_sim():
         None
     """
     # Get version of CSSTSim Package
-    __version__ = get_distribution("CSSTSim").version
+    __version__ = version("CSSTSim")
 
     # Get run datetime
     now = datetime.datetime.now()