Fix pypyNone interpreter spec when major version unspecified (#136)

* Demonstrate `pypyNone` interpreter spec when major version unspecified

* Fix `pypyNone` interpreter spec when major version unspecified

This resolves the following error that occurs
when the tox environment is simply named `pypy`:

```
pypy: skipped because could not find python interpreter with spec(s): pypyNone
```

src/tox_uv/_venv.py

@@ -183,7 +183,12 @@ def create_python_env(self) -> None:
             version_spec = sys.executable
         else:
             uv_imp = "" if (imp and imp == "cpython") else imp
-            version_spec = f"{uv_imp or ''}{base.major}.{base.minor}" if base.minor else f"{uv_imp or ''}{base.major}"
+            if not base.major:
+                version_spec = f"{uv_imp or ''}"
+            elif not base.minor:
+                version_spec = f"{uv_imp or ''}{base.major}"
+            else:
+                version_spec = f"{uv_imp or ''}{base.major}.{base.minor}"
 
         cmd: list[str] = [self.uv, "venv", "-p", version_spec, "--allow-existing"]
         if self.options.verbosity > 3:  # noqa: PLR2004

tests/test_tox_uv_venv.py

@@ -12,6 +12,7 @@
 from typing import TYPE_CHECKING, get_args
 
 import pytest
+import tox.tox_env.errors
 
 from tox_uv._venv import PythonPreference
 
@@ -54,6 +55,48 @@ def test_uv_venv_spec_major_only(tox_project: ToxProjectCreator) -> None:
     result.assert_success()
 
 
+@pytest.mark.parametrize(
+    ("pypy", "expected_uv_pypy"),
+    [
+        ("pypy", "pypy"),
+        ("pypy9", "pypy9"),
+        ("pypy999", "pypy9.99"),
+        ("pypy9.99", "pypy9.99"),
+    ],
+)
+def test_uv_venv_spec_pypy(
+    capfd: pytest.CaptureFixture[str],
+    tox_project: ToxProjectCreator,
+    pypy: str,
+    expected_uv_pypy: str,
+) -> None:
+    """Validate that major and minor versions are correctly applied to implementations.
+
+    This test prevents a regression that occurred when the testenv name was "pypy":
+    the uv runner was asked to use "pypyNone" as the Python version.
+
+    The test is dependent on what PyPy interpreters are installed on the system;
+    if any PyPy is available then the "pypy" value will not raise a Skip exception,
+    and STDOUT will be captured in `result.out`.
+
+    However, it is expected that no system will have PyPy v9.x installed,
+    so STDOUT must be read from `capfd` after the Skip exception is caught.
+
+    Since it is unknown whether any PyPy interpreter will be installed,
+    the `else` block's branch coverage is disabled.
+    """
+
+    project = tox_project({"tox.ini": f"[tox]\nenv_list = {pypy}"})
+    try:
+        result = project.run("config", "-vv")
+    except tox.tox_env.errors.Skip:
+        stdout, _ = capfd.readouterr()
+    else:  # pragma: no cover (PyPy might not be available on the system)
+        stdout = result.out
+    assert "pypyNone" not in stdout
+    assert f"-p {expected_uv_pypy} " in stdout
+
+
 @pytest.fixture
 def other_interpreter_exe() -> pathlib.Path:  # pragma: no cover
     """Returns an interpreter executable path that is not the exact same as `sys.executable`.