test_datastore_linesstats.py: Move importorskip earlier

The pytest.importorskip must be before local imports, that might to try
to import module like 'panel' that is not available (e.g. in GitHub
Action environment).

While at it, import 'panel' as 'pn' (with .importorskip), like in
other places.

tests/test_datastore_linesstats.py

@@ -1,14 +1,14 @@
 import pytest
 
+hv = pytest.importorskip("holoviews")
+param = pytest.importorskip("param")
+pn = pytest.importorskip("panel")
+
 from diffinsights_web.datastore import find_dataset_dir
 from diffinsights_web.datastore.timeline import TimelineDataStore
 from diffinsights_web.datastore.linesstats import LinesStatsDataStore, sorted_changed_files, \
     limit_count_to_selected_files, path_to_dirs_only_counter, reduce_sankey_from_tail, reduce_sankey_thin_out
 
-hv = pytest.importorskip("holoviews")
-param = pytest.importorskip("param")
-panel = pytest.importorskip("panel")
-
 
 def test_timeseries_file_no_such_file():
     data_store = LinesStatsDataStore(