Update xonsh with Python 3.8

Author: niess

appimage/recipes/xonsh.sh

@@ -1,4 +1,3 @@
-export PYTHON_VERSION="3.7.5"
+export PYTHON_VERSION="3.8.0"
 export PIP_REQUIREMENTS="xonsh prompt_toolkit gnureadline Pygments"
 export PYTHON_ENTRYPOINT='"-u" "-c" "from xonsh.main import main; main()"'
-export LD_LIBRARY_PATH="AppDir/usr/python/lib/python3.7/site-packages/.libsgnureadline:${LD_LIBRARY_PATH}"