Fix pandas and pytest deprecation warnings

[hagenw]

Changes indexing of dataframes and series to be in line with the latest pandas deprecation warnings, e.g. avoid chnaged indexing like df[col][row] and use df.loc[row, col] instead.

It also avoids using pytest.warns(None, ...) as this is also marked as deprecated by pytest.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (12ad852) 100.0% compared to head (bee2928) 100.0%.
Report is 1 commits behind head on main.

Additional details and impacted files

Files	Coverage Δ
audb/core/load.py	100.0% <100.0%> (ø)