Deprecate basemap (#243)

* Deprecate basemap. Basemap will be deprecated in the next major release (v2.0). See #243 for context.

Author: cvanelteren

ultraplot/figure.py

@@ -824,6 +824,10 @@ def _parse_backend(backend=None, basemap=None):
         Handle deprecation of basemap and cartopy package.
         """
         # Basemap is currently being developed again so are removing the deprecation warning
+        if backend == "basemap":
+            warnings._warn_ultraplot(
+                f"{backend=} will be deprecated in next major release (v2.0). See https://github.com/Ultraplot/ultraplot/pull/243"
+            )
         return backend
 
     def _parse_proj(