CI(Travis): Set python version to 3.10 through pyenv global (#4731)

* CI(Travis): Set pyenv global to an already installed Python version

Might become unavailable, it will remain dependant on what Travis installs in their images

.travis.yml

@@ -31,6 +31,11 @@ env:
     - GRASS_EXTRA_CXXFLAGS="-Werror -fPIC -Wfatal-errors"
 
 before_install:
+  # Show available versions if ever pyenv global fails
+  - pyenv versions
+  - pyenv global 3.10
+  # Show that the selected version is correctly set
+  - pyenv versions
   - ./.travis/$TRAVIS_OS_NAME.before_install.sh
 
 install: