py38 functional tests run forever

The run step freezes after bluepymm.select_combos: test select_combos based on example simple1 test is executed.

Here is the last log produced, nothing else gets written after this log.

```
bluepymm.select_combos: test select_combos based on example simple1 ... /home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:379: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
ext_neurondb = ext_neurondb.append(emodel_ext_neurondb_rows)
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:381: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
megate_passed_all = megate_passed_all.append(
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:383: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
median_scores = median_scores.append(emodel_median_scores)
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:384: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
passed_combos = passed_combos.append(emodel_passed_combos)
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:379: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
ext_neurondb = ext_neurondb.append(emodel_ext_neurondb_rows)
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:381: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
megate_passed_all = megate_passed_all.append(
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:383: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
median_scores = median_scores.append(emodel_median_scores)
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:384: FutureWarning: The frame.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
passed_combos = passed_combos.append(emodel_passed_combos)
/home/runner/work/BluePyMM/BluePyMM/bluepymm/select_combos/reporting.py:313: UserWarning: Tight layout not applied. The bottom and top margins cannot be made large enough to accommodate all axes decorations.
plt.tight_layout()
ok
```

Beginne mit dem Funktionstest für select_combos auf Grundlage des Beispiels simple1 und untersuche anschließend bluepymm/select_combos/reporting.py in der Nähe der Zeilen 313 und 379-384. Führe den Lauf erneut aus und ermittle, was nach der letzten Log-Ausgabe passiert; fertig ist es, wenn der Test abgeschlossen wird, anstatt einzufrieren.

Vom Indexierungsmodell aus dem Issue-Text verfasst.