add xESMF workaround

docs/conf.py

@@ -16,12 +16,20 @@
 # directory, add these directories to sys.path here. If the directory is
 # relative to the documentation root, use os.path.abspath to make it
 # absolute, like shown here.
-#
+from pathlib import Path
+
 import os
 import sys
 sys.path.insert(0, os.path.abspath('..'))
 
-import xhydro
+import xhydro  # noqa
+
+
+if os.environ.get("READTHEDOCS") and "ESMFMKFILE" not in os.environ:
+    # RTD doesn't activate the env, and esmpy depends on an env var set there
+    # We assume the `os` package is in {ENV}/lib/pythonX.X/os.py
+    # See conda-forge/esmf-feedstock#91 and readthedocs/readthedocs.org#4067
+    os.environ["ESMFMKFILE"] = str(Path(os.__file__).parent.parent / "esmf.mk")
 
 # -- General configuration ---------------------------------------------