Under tox, sys._base_executable is not an actual file for 3.11.0a3.  It was fine in 3.11.0a2.

To reproduce:

--- 8< --------------------
# tox.ini
[tox]
envlist = py{310,311a2,311}
skipsdist = True

[testenv]
commands =
    python -c "import sys,os.path; print(e := sys._base_executable); print(os.path.exists(e))"

[testenv:py311a2]
# This is the path to 3.11.0a2 if you have it.
basepython = /usr/local/pyenv/pyenv/versions/3.11.0a2/bin/python3
----------------------------

Create a new Python 3.8 virtualenv, and install latest tox (3.24.4 for me).

Then run "tox".  I see:

--------------------------------------------------------------------------------
py310 create: /Users/nedbatchelder/coverage/lab/fix-3.11a3/.tox/py310
py310 run-test-pre: PYTHONHASHSEED='534434199'
py310 run-test: commands[0] | python -c 'import sys,os.path; print(e := sys._base_executable); print(os.path.exists(e))'
/Users/nedbatchelder/coverage/lab/fix-3.11a3/.tox/py310/bin/python
True
py311a2 create: /Users/nedbatchelder/coverage/lab/fix-3.11a3/.tox/py311a2
py311a2 run-test-pre: PYTHONHASHSEED='534434199'
py311a2 run-test: commands[0] | python -c 'import sys,os.path; print(e := sys._base_executable); print(os.path.exists(e))'
/Users/nedbatchelder/coverage/lab/fix-3.11a3/.tox/py311a2/bin/python
True
py311 create: /Users/nedbatchelder/coverage/lab/fix-3.11a3/.tox/py311
py311 run-test-pre: PYTHONHASHSEED='534434199'
py311 run-test: commands[0] | python -c 'import sys,os.path; print(e := sys._base_executable); print(os.path.exists(e))'
/usr/local/pyenv/pyenv/versions/3.11.0a3/python
False
_________________________________________________________ summary _________________________________________________________
  py310: commands succeeded
  py311a2: commands succeeded
  py311: commands succeeded
  congratulations :)
--------------------------------------------------------------------------------

This came to my attention because the coverage.py test suite uses "python -m venv" to create environments. They worked under 3.11.0a2, but failed under a3.  I tracked it down to a difference in sys._base_executable.

I couldn't see a difference in those values without tox, but I'm not sure why it changes the results.