Completely disable outside python

```julia
Package Version Build Channel Size
───────────────────────────────────────────────────────────────────────
Reinstall:
───────────────────────────────────────────────────────────────────────

o libstdcxx-ng 11.3.0 h239ccf8_18 conda-forge Cached
o python 3.10.6 h582c2e5_0_cpython conda-forge Cached

Summary:

Reinstall: 1 packages

Total download: 0 B

───────────────────────────────────────────────────────────────────────

Transaction starting
Changing libstdcxx-ng-11.3.0-h239ccf8_18 ==> libstdcxx-ng-11.3.0-h239ccf8_18
Changing python-3.10.6-h582c2e5_0_cpython ==> python-3.10.6-h582c2e5_0_cpython
Transaction finished
Python path configuration:
PYTHONHOME = '/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt'
PYTHONPATH = '/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/lib/python3.9/site-packages/itk:/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/python:/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/lib:/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/lib/python3.9/site-packages'
program name = '/home/jiling/.julia/dev/WVZAnalysis/.CondaPkg/env/bin/python'
isolated = 0
environment = 1
user site = 1
import site = 1
sys._base_executable = '/home/jiling/.julia/dev/WVZAnalysis/.CondaPkg/env/bin/python'
sys.base_prefix = '/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt'
sys.base_exec_prefix = '/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt'
sys.platlibdir = 'lib'
sys.executable = '/home/jiling/.julia/dev/WVZAnalysis/.CondaPkg/env/bin/python'
sys.prefix = '/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt'
sys.exec_prefix = '/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt'
sys.path = [
'/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/lib/python3.9/site-packages/itk',
'/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/python',
'/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/lib',
'/cvmfs/sft-nightlies.cern.ch/lcg/views/dev4/Wed/x86_64-centos7-gcc11-opt/lib/python3.9/site-packages',
'/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt/lib/python310.zip',
'/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt/lib/python3.10',
'/cvmfs/sft.cern.ch/lcg/releases/Python/3.9.12-9a1bc/x86_64-centos7-gcc11-opt/lib/python3.10/lib-dynload',
]
Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
```

The outside environment setup a pretty intrusive set of environment variables, how to protect against it?

No source file or test is named. Start by examining the reported Python path configuration and the interaction with external PYTHONHOME and PYTHONPATH variables. Done should mean that an outside environment cannot redirect the Python environment used by PythonCall, with a regression test covering the reported contamination.

Written by the indexing model from the issue text.