fix mypy and format

Author: chelsea-lin

bigframes/core/array_value.py

@@ -21,7 +21,6 @@
 import warnings
 
 import google.cloud.bigquery
-import pandas
 import pyarrow as pa
 
 import bigframes.core.expression as ex

tests/system/small/test_dataframe.py

@@ -3765,9 +3765,9 @@ def test_at_no_duplicate(scalars_df_index, scalars_pandas_df_index):
 
 
 def test_setitem_w_timestamp_none():
-    b_df = bpd.DataFrame({'rowindex': [1, 2, 3]})
-    b_df['temp_timestamp'] = pd.Timestamp(ts_input=None, unit="us", tz="utc")
-    assert b_df['temp_timestamp'].dtype == "timestamp[us][pyarrow]"
+    b_df = bpd.DataFrame({"rowindex": [1, 2, 3]})
+    b_df["temp_timestamp"] = pd.Timestamp(ts_input=pd.NaT, unit="us", tz="utc")  # type: ignore
+    assert b_df["temp_timestamp"].dtype == "timestamp[us][pyarrow]"
 
 
 def test_loc_setitem_bool_series_scalar_new_col(scalars_dfs):
@@ -5720,4 +5720,4 @@ def test_agg_with_dict_containing_non_existing_col_raise_key_error(scalars_dfs):
     }
 
     with pytest.raises(KeyError):
-        bf_df.agg(agg_funcs)
+        bf_df.agg(agg_funcs)