feat(jenkins): modernize pipeline for Python 3 and quickstart-tutorials

- Replace Docker-based agents with 'agent any' for compatibility
  with jenkins-docs/quickstart-tutorials environment
- Update Build stage to use python3 instead of python
- Add skipStagesAfterUnstable option
- Add stash for compiled results
- Remove deprecated Docker images (python:2-alpine, qnib/pytest,
  cdrx/pyinstaller-linux:python2)

This change aligns the reference Jenkinsfile with the quickstart-tutorials
infrastructure which provides a Python agent with Python 3, pytest,
and pyinstaller pre-installed.

Fixes #85

Author: gounthar

jenkins/Jenkinsfile

@@ -1,22 +1,16 @@
 pipeline {
-    agent none
+    agent any
+    options {
+        skipStagesAfterUnstable()
+    }
     stages {
         stage('Build') {
-            agent {
-                docker {
-                    image 'python:2-alpine'
-                }
-            }
             steps {
-                sh 'python -m py_compile sources/add2vals.py sources/calc.py'
+                sh 'python3 -m py_compile sources/add2vals.py sources/calc.py'
+                stash(name: 'compiled-results', includes: 'sources/*.py*')
             }
         }
         stage('Test') {
-            agent {
-                docker {
-                    image 'qnib/pytest'
-                }
-            }
             steps {
                 sh 'py.test --verbose --junit-xml test-reports/results.xml sources/test_calc.py'
             }
@@ -27,11 +21,6 @@ pipeline {
             }
         }
         stage('Deliver') {
-            agent {
-                docker {
-                    image 'cdrx/pyinstaller-linux:python2'
-                }
-            }
             steps {
                 sh 'pyinstaller --onefile sources/add2vals.py'
             }