Fail on mismatched python spec attributes (#2824)

Closes #2754

docs/changelog/2754.bugfix.rst

@@ -0,0 +1,2 @@
+Setting ``[testenv] basepython = python3`` will no longer override the Python interpreter version requested by a factor,
+such as ``py311`` - by :user:`stephenfin`.

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"]),