fix mypy errors

Author: tswast

bigframes/formatting_helpers.py

@@ -105,8 +105,14 @@ def progress_callback(
     """Displays a progress bar while the query is running"""
     global current_display, current_display_id, previous_display_html
 
-    import bigframes._config
-    import bigframes.core.events
+    try:
+        import bigframes._config
+        import bigframes.core.events
+    except ImportError:
+        # Since this gets called from __del__, skip if the import fails to avoid
+        # ImportError: sys.meta_path is None, Python is likely shutting down.
+        # This will allow cleanup to continue.
+        return
 
     progress_bar = bigframes._config.options.display.progress_bar
 

tests/system/small/test_session_as_bpd.py

@@ -140,10 +140,9 @@ def test_to_datetime(session: bigframes.session.Session):
 
 def test_to_timedelta(session: bigframes.session.Session):
     offsets = np.arange(5)
-    unit = "s"
 
-    bpd_result = bpd.to_timedelta(offsets, unit=unit)
-    session_result = session.to_timedelta(offsets, unit=unit)
+    bpd_result = bpd.to_timedelta(offsets, unit="s")
+    session_result = session.to_timedelta(offsets, unit="s")
 
     global_session = bpd.get_global_session()
     assert global_session is not session