[SPARK-37704] Update mypy version to 0.920

### What changes were proposed in this pull request?

This PR updates mypy version in GitHub workflow to 0.920.

Additionally, it fixes minor typing issue not detected by 0.910.

### Why are the changes needed?

To keep annotations up to date with typing tools.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing tests.

Closes #34972 from zero323/SPARK-37704.

Authored-by: zero323 <[email protected]>
Signed-off-by: zero323 <[email protected]>

Author: zero323

.github/workflows/build_and_test.yml

@@ -477,7 +477,7 @@ jobs:
         #   See also https://github.com/sphinx-doc/sphinx/issues/7551.
         # Jinja2 3.0.0+ causes error when building with Sphinx.
         #   See also https://issues.apache.org/jira/browse/SPARK-35375.
-        python3.9 -m pip install 'flake8==3.9.0' pydata_sphinx_theme 'mypy==0.910' numpydoc 'jinja2<3.0.0' 'black==21.5b2'
+        python3.9 -m pip install 'flake8==3.9.0' pydata_sphinx_theme 'mypy==0.920' numpydoc 'jinja2<3.0.0' 'black==21.5b2'
         # TODO Update to PyPI
         python3.9 -m pip install git+https://github.com/typeddjango/pytest-mypy-plugins.git@b0020061f48e85743ee3335bd62a3a608d17c6bd
     - name: Install R linter dependencies and SparkR

python/pyspark/pandas/indexing.py

@@ -553,7 +553,7 @@ def __getitem__(self, key: Any) -> Union["Series", "DataFrame"]:
 
         psdf_or_psser: Union[DataFrame, Series]
         if returns_series:
-            psdf_or_psser = first_series(psdf)
+            psdf_or_psser = cast(Series, first_series(psdf))
             if series_name is not None and series_name != psdf_or_psser.name:
                 psdf_or_psser = psdf_or_psser.rename(series_name)
         else: