Figure.psconvert: Ignore the unrecognized 'metadata' parameter added by pytest-mpl v0.17.0

[seisman]

Address #3043 (comment):

After pinning pandas and xarray with Python 3.10, now we have many failures on macOS/Windows. See https://github.com/GenericMappingTools/pygmt/actions/runs/7946102470/job/21694725831. Need to find out why.

pytest-mpl=0.17.0 now adds a metadata key into our savefig call (added in https://github.com/matplotlib/pytest-mpl/pull/196/files I think), and this is not handled by psconvert. We'll need to ignore that metadata key somehow.