Use python3 in more shebang lines

This is needed to run the python unit tests on cheyenne without
explicitly pointing to python3.

There are still more shebang instances throughout CTSM that I have not
yet changed, because I'm not positive if they should be changed
yet. (buildnml and buildlib should probably be changed in conjunction
with others through CESM, and I'm not sure about the usage of the
contrib tools.)

python/Makefile

@@ -8,7 +8,7 @@ debug = not-set
 ifneq ($(python), not-set)
     PYTHON=$(python)
 else
-    PYTHON=python
+    PYTHON=python3
 endif
 
 ifneq ($(debug), not-set)

python/README.md

@@ -14,7 +14,7 @@ thing, but support different options:
 
    You can specify a few arguments to this:
 
-   - python version: `make python=python3 test`
+   - python version: `make python=python3.9 test` (defaults to `python3`; you should expect errors if trying to run with python2)
    - verbose: `make verbose=true test`
    - debug: `make debug=true test`
 

python/ctsm/test/joblauncher/test_unit_job_launcher_no_batch.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for job_launcher_no_batch
 """

python/ctsm/test/test_sys_lilac_build_ctsm.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """System tests for lilac_build_ctsm
 

python/ctsm/test/test_unit_lilac_build_ctsm.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for lilac_build_ctsm
 """

python/ctsm/test/test_unit_lilac_make_runtime_inputs.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for lilac_make_runtime_inputs
 """

python/ctsm/test/test_unit_machine.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for machine
 """

python/ctsm/test/test_unit_path_utils.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for path_utils
 """

python/ctsm/test/test_unit_run_sys_tests.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for run_sys_tests
 """

python/ctsm/test/test_unit_utils.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 """Unit tests for utils
 """

python/run_ctsm_py_tests

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Driver for running the unit tests of the python code
 
 We use this rather than simply relying on 'python -m unittest discover' so we can do some

run_sys_tests

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """Driver for running CTSM system tests"""
 
 import os