Fix deprecated pygmt maskvalues parameter in grdlandmask call

[Copilot]

pygmt.grdlandmask deprecated the maskvalues parameter in v0.18.0 (to be removed in v0.20.0) in favour of mask_values.

Changes

• pygmt_helper/plotting.py: Rename maskvalues → mask_values in the pygmt.grdlandmask call

# Before
land_mask = pygmt.grdlandmask(
    region=...,
    spacing=grid_spacing,
    maskvalues=[0, 1, 1, 1, 1],  # deprecated
    resolution="f",
)

# After
land_mask = pygmt.grdlandmask(
    region=...,
    spacing=grid_spacing,
    mask_values=[0, 1, 1, 1, 1],
    resolution="f",
)

[lispandfound]

@copilot the tests are failing.

[Copilot]

@copilot the tests are failing.

The CI was using pygmt 0.16.0 (pinned in uv.lock), which predates the mask_values parameter. Fixed in commit Update pygmt to >=0.18.0 and bump requires-python to >=3.12 by:

• Adding pygmt>=0.18.0 constraint to requirements.txt
• Updating requires-python to >=3.12 (since pygmt 0.18.0 requires Python 3.12+)
• Regenerating uv.lock with pygmt 0.18.0