Switch language from None to "en" in docs build (#11133)

Needed for Sphinx 5 compatibility. Should fix the following warning occurring in doc builds.

```
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).
```

xref: sphinx-doc/sphinx#10481

Authors:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - https://github.com/jakirkham
  - Ray Douglass (https://github.com/raydouglass)

URL: #11133

docs/cudf/source/conf.py

@@ -91,7 +91,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = "en"
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.

python/cudf/cudf/tests/test_parquet.py

@@ -2552,9 +2552,9 @@ def test_parquet_reader_zstd_compression(datadir):
         pytest.mark.xfail(reason="zstd support is not enabled")
 
 
-def test_read_parquet_multiple_files(datadir):
-    df_1_path = datadir / "df_1.parquet"
-    df_2_path = datadir / "df_2.parquet"
+def test_read_parquet_multiple_files(tmpdir):
+    df_1_path = tmpdir / "df_1.parquet"
+    df_2_path = tmpdir / "df_2.parquet"
     df_1 = cudf.DataFrame({"id": range(100), "a": [1] * 100})
     df_1.to_parquet(df_1_path)