Fix outdated min version of virtualenv

[kajinamit]

The recent 4.46.0 release introduced the feature[1] which requires virtualenv>=20.39.0 .

[1] #3815

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[stephenfin]

@gaborbernat Would it be possible to yank the broken releases?

❯ cat constraints.txt
virtualenv===20.38.0

❯ cat tox.ini
[testenv]
deps =
  requests

❯ virtualenv .venv
❯ source .venv/bin/activate
❯ pip install -c constraint.txt tox
Collecting tox
  Using cached tox-4.46.3-py3-none-any.whl.metadata (3.6 kB)
Collecting cachetools>=7.0.1 (from tox)
  Using cached cachetools-7.0.1-py3-none-any.whl.metadata (5.6 kB)
Collecting colorama>=0.4.6 (from tox)
  Using cached colorama-0.4.6-py2.py3-none-any.whl.metadata (17 kB)
Collecting filelock>=3.24.3 (from tox)
  Using cached filelock-3.24.3-py3-none-any.whl.metadata (2.0 kB)
Collecting packaging>=26 (from tox)
  Using cached packaging-26.0-py3-none-any.whl.metadata (3.3 kB)
Collecting platformdirs>=4.9.2 (from tox)
  Using cached platformdirs-4.9.2-py3-none-any.whl.metadata (4.7 kB)
Collecting pluggy>=1.6 (from tox)
  Using cached pluggy-1.6.0-py3-none-any.whl.metadata (4.8 kB)
Collecting pyproject-api>=1.10 (from tox)
  Using cached pyproject_api-1.10.0-py3-none-any.whl.metadata (2.7 kB)
INFO: pip is looking at multiple versions of tox to determine which version is compatible with other requirements. This could take a while.
Collecting tox
  Using cached tox-4.46.2-py3-none-any.whl.metadata (3.6 kB)
Collecting virtualenv>=20.38 (from tox)
  Using cached virtualenv-20.38.0-py3-none-any.whl.metadata (5.1 kB)
Collecting distlib<1,>=0.3.7 (from virtualenv>=20.38->tox)
  Using cached distlib-0.4.0-py2.py3-none-any.whl.metadata (5.2 kB)
Using cached tox-4.46.2-py3-none-any.whl (201 kB)
Using cached cachetools-7.0.1-py3-none-any.whl (13 kB)
Using cached colorama-0.4.6-py2.py3-none-any.whl (25 kB)
Using cached filelock-3.24.3-py3-none-any.whl (24 kB)
Using cached packaging-26.0-py3-none-any.whl (74 kB)
Using cached platformdirs-4.9.2-py3-none-any.whl (21 kB)
Using cached pluggy-1.6.0-py3-none-any.whl (20 kB)
Using cached pyproject_api-1.10.0-py3-none-any.whl (13 kB)
Using cached virtualenv-20.38.0-py3-none-any.whl (5.8 MB)
Using cached distlib-0.4.0-py2.py3-none-any.whl (469 kB)
Installing collected packages: distlib, pluggy, platformdirs, packaging, filelock, colorama, cachetools, virtualenv, pyproject-api, tox
Successfully installed cachetools-7.0.1 colorama-0.4.6 distlib-0.4.0 filelock-3.24.3 packaging-26.0 platformdirs-4.9.2 pluggy-1.6.0 pyproject-api-1.10.0 tox-4.46.2 virtualenv-20.38.0

[notice] A new release of pip is available: 25.1.1 -> 26.0.1
[notice] To update, run: pip install --upgrade pip

❯ tox --version
4.46.2 from /tmp/foo/.venv/lib/python3.14/site-packages/tox/__init__.py

❯ tox -e py310
py310: internal error
Traceback (most recent call last):
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/session/cmd/run/single.py", line 51, in _evaluate
    tox_env.setup()
    ~~~~~~~~~~~~~^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/api.py", line 299, in setup
    self._setup_env()
    ~~~~~~~~~~~~~~~^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/runner.py", line 145, in _setup_env
    super()._setup_env()
    ~~~~~~~~~~~~~~~~~~^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/api.py", line 279, in _setup_env
    self.ensure_python_env()
    ~~~~~~~~~~~~~~~~~~~~~~^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/api.py", line 283, in ensure_python_env
    conf = self.python_cache()
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/virtual_env/api.py", line 95, in python_cache
    base = super().python_cache()
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/api.py", line 320, in python_cache
    "version_info": list(self.base_python.version_info),
                         ^^^^^^^^^^^^^^^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/api.py", line 326, in base_python
    base_pythons: list[str] = self.conf["base_python"]
                              ~~~~~~~~~^^^^^^^^^^^^^^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/config/sets.py", line 150, in __getitem__
    return self.load(item)
           ~~~~~~~~~^^^^^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/config/sets.py", line 162, in load
    return config_definition.__call__(self._conf, self.loaders, ConfigLoadArgs(chain, self.name, self.env_name))  # noqa: PLC2801
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/config/of_type.py", line 123, in __call__
    value = self.post_process(value)
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/api.py", line 95, in validate_base_python
    return self._validate_base_python(self.name, value, self.core["ignore_base_python_conflict"])
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/api.py", line 224, in _validate_base_python
    if any(
       ^^^
  File "/tmp/foo/.venv/lib/python3.14/site-packages/tox/tox_env/python/api.py", line 227, in <genexpr>
    if getattr(spec_name, key) is not None
       ~~~~~~~^^^^^^^^^^^^^^^^
AttributeError: 'PythonSpec' object has no attribute 'machine'
  py310: FAIL code 2 (0.00 seconds)
  evaluation failed :( (0.03 seconds)

This affects 4.46.0 - 4.46.2.