Remove pandas warning when using bullseye version of pandas

The following warning is shown when using OnionPerf's visualize mode with version 0.25.3 of pandas (contained in Debian bullseye). It is not shown when using buster's version of pandas. If there's an easy way to remove the warning when using either version, let's do it.

    /usr/lib/python3/dist-packages/pandas/plotting/_matplotlib/converter.py:103: FutureWarning: Using an implicitly registered datetime converter for a matplotlib plotting method. The converter was registered by pandas on import. Future versions of pandas will require you to explicitly register matplotlib converters.

    To register the converters:
    	>>> from pandas.plotting import register_matplotlib_converters
    	>>> register_matplotlib_converters()
      warnings.warn(msg, FutureWarning)