Fallback on PYTHON path in Makefile

- Change default PYTHON in Makefile to be ranked choice
- Fix `PYTHON_VERSION` target that expects just a word
- Use native GNU Make `$(subst ,,)` instead of `sed`
- Add 'version-for-buildyml' target to simplify ci

If I understand correctly, this change should make
'$(PYTHON)' work how we want it to everywhere. Before
this change, on develpers' machines that don't have
a 'python3.9' in their path, make would fail. With this
change, we will prefer python3.9 if it's available, but
we'll take python3 otherwise.

tools/ansible/build.yml

@@ -4,8 +4,7 @@
   gather_facts: true
   tasks:
     - name: Get version from SCM if not explicitly provided
-      shell: |
-        make print-VERSION | cut -d + -f -1
+      command: make version-for-buildyml
       args:
         chdir: '../../'
       register: scm_version