Fix ESMFMKFILE env variable not set in RTD build (#577)

docs/conf.py

@@ -19,10 +19,18 @@
 #
 import os
 import sys
+from pathlib import Path
 from typing import Dict
 
 import sphinx_autosummary_accessors
 
+# A workaround that sets the "ESMFMKFILE" env variable for the Read The Docs
+# build to work. Read The Docs does not activate the conda environment which
+# causes "ESMFMKFILE" to not be set (required by `esmpy` and `xesmf`).
+# Source: https://github.com/conda-forge/esmf-feedstock/issues/91
+if os.environ.get("READTHEDOCS") and "ESMFMKFILE" not in os.environ:
+    os.environ["ESMFMKFILE"] = str(Path(os.__file__).parent.parent / "esmf.mk")
+
 sys.path.insert(0, os.path.abspath(".."))  # noqa: I001, I003
 import xcdat  # noqa: I001, E402