CI: Change tilemap source from Stamen to OpenStreetMap

[weiji14]

Description of proposed changes

Change tilemap source from contextily's default Stamen Terrain to OpenStreetmap's Standard tile layer (osm-carto, see https://wiki.openstreetmap.org/wiki/Standard_tile_layer).

References:

• https://contextily.readthedocs.io/en/latest/providers_deepdive.html
• https://wiki.openstreetmap.org/w/index.php?title=Standard_tile_layer&redirect=no
• https://operations.osmfoundation.org/policies/tiles/

Addresses #2699

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.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[github-actions]

Summary of changed images

This is an auto-generated report of images that have changed on the DVC remote

Status	Path
modified	pygmt/tests/baseline/test_tilemap_no_clip_False.png
modified	pygmt/tests/baseline/test_tilemap_no_clip_True.png
modified	pygmt/tests/baseline/test_tilemap_ogc_wgs84.png
modified	pygmt/tests/baseline/test_tilemap_web_mercator.png

Image diff(s)

Added images

Modified images

Path	Old	New
test_tilemap_no_clip_False.png
test_tilemap_no_clip_True.png
test_tilemap_ogc_wgs84.png
test_tilemap_web_mercator.png

Report last updated at commit 0e04187

[weiji14]

Some small differences in generated images due to differing ghostscript versions. CI is using gs 9.54, whereas I was using gs 9.56 in 49cd040. Trying with gs 10.02 (xref #2694) at 7ee18ab.