Reduce Python 3 detection

With AC-195, macOS environments no longer need to search for other
Python 3 deployments that might have `virtualenv`.

References:
* [AC-195] Use Python venv instead of virtualenv to build virtual
  environments for CI

Signed-off-by: Alex Nelson <[email protected]>

Author: ajnelson-nist

Makefile

@@ -13,7 +13,7 @@
 
 SHELL := /bin/bash
 
-PYTHON3 ?= $(shell which python3.9 2>/dev/null || which python3.8 2>/dev/null || which python3.7 2>/dev/null || which python3.6 2>/dev/null || which python3)
+PYTHON3 ?= python3
 
 case_version := $(shell $(PYTHON3) case_utils/ontology/version_info.py)
 ifeq ($(case_version),)

tests/Makefile

@@ -15,7 +15,7 @@ SHELL := /bin/bash
 
 top_srcdir := $(shell cd .. ; pwd)
 
-PYTHON3 ?= $(shell which python3.9 2>/dev/null || which python3.8 2>/dev/null || which python3.7 2>/dev/null || which python3.6 2>/dev/null || which python3)
+PYTHON3 ?= python3
 
 all: \
   all-case_utils