Fix Cartopy issue and require >= 0.18

[santisoler]

Unpin Cartopy because Cartopy 0.17.0 has a compatibility issue with Matplotlib > 3.3.0.
Remove the unneeded tight_layout that was causing errors.
Replace the filled polygons used for plotting land and ocean for the coastlines.
See: SciTools/cartopy#1615

Reminders:

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[santisoler]

Now there's a new error after running tight_layout() on the examples.
And the plotting issue on Cartopy (#269) is not yet solved.

[leouieda]

I would propose:

1. Unpin cartopy
2. Remove the tight_layout since it doesn't do anything when you have cartopy plots
3. Stop plotting filled polygons and just do coastlines instead. Plots are not as nice but at least they work.