Fail on mismatched python spec attributes

The following is intended to be a correct tox configuration:

  [tox]
  min_version = 4.1
  env_list = py{38,39,310,311},docs

  [testenv]
  base_python = python3

  [testenv:docs]
  commands =
    sphinx-build ...

The goal of this is to use 'python3' (i.e. the value of '[testenv]
base_python') for all environments except those with specific 'pyXX'
factors in them. This helps eliminate issues where environments that do
not specify a 'pyXX' factor run with different Python versions in
different environments.

An earlier bug, #477 [1], prevented us from doing this. Due to #477, the
interpreter version indicated by '[testenv] base_python' (or rather
'[testenv] basepython' - the underscore-separated variant was only
introduced in tox 4) would override the value indicated by the 'pyXX'
factor. This was resolved with a PR, #841 [2], which started warning
users if they were unintentionally running against the wrong interpreter
and introduced the 'ignore_basepython_conflict' value to opt into the
correct behavior.

Unfortunately, this has been broken in the move to tox 4. Currently,
running with the above configuration will result in 'python3' being used
for every environment. This is clearly incorrect. This issue stems from
an errant bit of logic. When comparing interpreter versions as part of
the '_validate_base_python', we ignore various attributes if they are
unset on either '[testenv] base_python' or the 'pyXX' factor. This
allows e.g. 'python3' to match 'python3.8', which isn't correct. Correct
this by insisting on matching on all attributes that are set on the
factor.

[1] #477
[2] #841

Signed-off-by: Stephen Finucane <stephen@that.guru>
Closes: #2754

src/tox/tox_env/python/api.py

@@ -154,7 +154,7 @@ def _validate_base_python(env_name: str, base_pythons: list[str], ignore_base_py
                 if any(
                     getattr(spec_base, key) != getattr(spec_name, key)
                     for key in ("implementation", "major", "minor", "micro", "architecture")
-                    if getattr(spec_base, key) is not None and getattr(spec_name, key) is not None
+                    if getattr(spec_name, key) is not None
                 ):
                     msg = f"env name {env_name} conflicting with base python {base_python}"
                     if ignore_base_python_conflict:

tests/tox_env/python/test_python_api.py

@@ -95,6 +95,7 @@ def test_base_python_env_no_conflict(env: str, base_python: list[str], ignore_co
         ("py3", ["py2"], ["py2"]),
         ("py38", ["py39"], ["py39"]),
         ("py38", ["py38", "py39"], ["py39"]),
+        ("py38", ["python3"], ["python3"]),
         ("py310", ["py38", "py39"], ["py38", "py39"]),
         ("py3.11.1", ["py3.11.2"], ["py3.11.2"]),
         ("py3-64", ["py3-32"], ["py3-32"]),