Fixed Travis runs with libxml2/py38

Signed-off-by: Andreas Maier <andreas.r.maier@gmx.de>

.travis.yml

@@ -130,34 +130,44 @@ matrix:
 #        - PACKAGE_LEVEL=latest
 
 #    - os: osx
+#      osx_image: xcode9.4  # macOS 10.13.6 with python2 @ 2.7.17
 #      language: generic
 #      python:
 #      env:
 #        - PYTHON=2
 #        - PACKAGE_LEVEL=minimum
 
 #    - os: osx
+#      osx_image: xcode9.4  # macOS 10.13.6 with python2 @ 2.7.17
 #      language: generic
 #      python:
 #      env:
 #        - PYTHON=2
 #        - PACKAGE_LEVEL=latest
 
 #    - os: osx
+#      osx_image: xcode12  # macOS 10.15.5 with python3 @ 3.7.7, upgraded to 3.8.3
 #      language: generic
 #      python:
 #      env:
 #        - PYTHON=3
 #        - PACKAGE_LEVEL=minimum
 
 #    - os: osx
+#      osx_image: xcode12  # macOS 10.15.5 with python3 @ 3.7.7, upgraded to 3.8.3
 #      language: generic
 #      python:
 #      env:
 #        - PYTHON=3
 #        - PACKAGE_LEVEL=latest
 
 before_install:
+
+  # Debug original Python versions on macOS
+  - sh -c "which python; python --version"
+  - sh -c "which python2; python2 --version"
+  - sh -c "which python3; python3 --version"
+
   - if [[ "$TRAVIS_BRANCH" == "manual-ci-run" ]]; then
       export _NEED_REBASE=true;
     fi