chore: nox doctests only in Python 3.12 (#1222)

* only doctest in 3.12 in nox

* skip writing file in doctests

Author: EdAbati

narwhals/dataframe.py

@@ -582,9 +582,9 @@ def write_parquet(self, file: str | Path | BytesIO) -> Any:
 
             We can then pass either pandas, Polars or PyArrow to `func`:
 
-            >>> func(df_pd)
-            >>> func(df_pl)
-            >>> func(df_pa)
+            >>> func(df_pd)  # doctest:+SKIP
+            >>> func(df_pl)  # doctest:+SKIP
+            >>> func(df_pa)  # doctest:+SKIP
         """
         self._compliant_frame.write_parquet(file)
 

noxfile.py

@@ -27,7 +27,9 @@ def run_common(session: Session, coverage_threshold: float) -> None:
         f"--cov-fail-under={coverage_threshold}",
         "--runslow",
     )
-    session.run("pytest", "narwhals", "--doctest-modules")
+
+    if session.python == "3.12":
+        session.run("pytest", "narwhals", "--doctest-modules")
 
 
 @nox.session(python=PYTHON_VERSIONS)  # type: ignore[misc]