MNT: Force Cartopy to always use the global context

We don't guarantee thread-safety within Cartopy anyways, so we should
just apply this every time.

lib/cartopy/__init__.py

@@ -6,6 +6,8 @@
 
 from ._version import version as __version__  # noqa: F401
 import tempfile
+from pyproj import set_use_global_context
+set_use_global_context(active=True)
 
 __document_these__ = ['config']