More protections against system python (alisw#5586)

There is no reason why we should have interference from system python
when building python itself.

python.sh

@@ -18,7 +18,7 @@ env:
   PYTHONPATH: "$PYTHON_ROOT/lib/python/site-packages"
 prefer_system: "(?!slc5|ubuntu)"
 prefer_system_check:
-  python3 -c 'import sys; import sqlite3; sys.exit(1 if sys.version_info < (3, 5) else 0)' && python3 -m pip --help > /dev/null && printf '#include "pyconfig.h"' | cc -c $(python3-config --includes) -xc -o /dev/null -; if [ $? -ne 0 ]; then printf "Python, the Python development packages, and pip must be installed on your system.\nUsually those packages are called python, python-devel (or python-dev) and python-pip.\n"; exit 1; fi
+  python3 -c 'import sys; import sqlite3; sys.exit(1 if sys.version_info < (3, 9) else 0)' && python3 -m pip --help > /dev/null && printf '#include "pyconfig.h"' | cc -c $(python3-config --includes) -xc -o /dev/null -; if [ $? -ne 0 ]; then printf "Python, the Python development packages, and pip must be installed on your system.\nUsually those packages are called python, python-devel (or python-dev) and python-pip.\n"; exit 1; fi
 ---
 rsync -av --exclude '**/.git' $SOURCEDIR/ $BUILDDIR/
 
@@ -28,6 +28,8 @@ export LIBFFI_ROOT
 # If the python installer finds another pip, it won't install the new one
 export PATH=$(echo $PATH | awk -v RS=':' -v ORS=':' '!/python/ {print}' | sed 's/:$//')
 unset PYTHONUSERBASE
+unset PYTHONHOME
+unset PYTHONPATH
 
 # The only way to pass externals to Python
 LDFLAGS=