Tools: environment_install: support Mint 20.1 Ulyssa

Support for Linux Mint 20.1 Ulyssa (currently last version)
based on Ubuntu 20.04 Focal implemented

Tools/environment_install/install-prereqs-ubuntu.sh

@@ -71,7 +71,7 @@ elif [ ${RELEASE_CODENAME} == 'disco' ]; then
 elif [ ${RELEASE_CODENAME} == 'eoan' ]; then
     SITLFML_VERSION="2.5"
     SITLCFML_VERSION="2.5"
-elif [ ${RELEASE_CODENAME} == 'focal' ]; then
+elif [ ${RELEASE_CODENAME} == 'focal' ] || [ ${RELEASE_CODENAME} == 'ulyssa' ]; then
     SITLFML_VERSION="2.5"
     SITLCFML_VERSION="2.5"
     PYTHON_V="python3"
@@ -181,7 +181,7 @@ sudo usermod -a -G dialout $USER
 echo "Done!"
 
 # Add back python symlink to python interpreter on Ubuntu >= 20.04
-if [ ${RELEASE_CODENAME} == 'focal' ]; then
+if [ ${RELEASE_CODENAME} == 'focal' ] || [ ${RELEASE_CODENAME} == 'ulyssa' ]; then
     BASE_PKGS+=" python-is-python3"
     SITL_PKGS+=" libpython3-stdlib" # for argparse
 elif [ ${RELEASE_CODENAME} == 'groovy' ]; then
@@ -196,7 +196,7 @@ if [[ $SKIP_AP_GRAPHIC_ENV -ne 1 ]]; then
   if [ ${RELEASE_CODENAME} == 'groovy' ]; then
     SITL_PKGS+=" python3-wxgtk4.0"
     SITL_PKGS+=" fonts-freefont-ttf libfreetype6-dev libjpeg8-dev libpng16-16 libportmidi-dev libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-ttf2.0-dev libsdl1.2-dev"  # for pygame
-  elif [ ${RELEASE_CODENAME} == 'focal' ]; then
+  elif [ ${RELEASE_CODENAME} == 'focal' ] || [ ${RELEASE_CODENAME} == 'ulyssa' ]; then
     SITL_PKGS+=" python3-wxgtk4.0"
     SITL_PKGS+=" fonts-freefont-ttf libfreetype6-dev libjpeg8-dev libpng16-16 libportmidi-dev libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-ttf2.0-dev libsdl1.2-dev"  # for pygame
   elif apt-cache search python-wxgtk3.0 | grep wx; then