Add workaround to Read The Docs ESMKFILE env var

docs/conf.py

@@ -19,10 +19,21 @@
 #
 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.
+# Source: https://github.com/conda-forge/esmf-feedstock/issues/91
+if os.environ.get("READTHEDOCS") and "ESMFMKFILE" not in os.environ:
+    # RTD doesn't activate the env, and esmpy depends on a 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")
+
 sys.path.insert(0, os.path.abspath(".."))  # noqa: I001, I003
 import xcdat  # noqa: I001, E402
 
@@ -53,7 +64,7 @@
 # with xesmf.
 # Related issue: https://github.com/readthedocs/readthedocs.org/issues/5512
 # Solution: https://docs.readthedocs.io/en/stable/faq.html#why-do-i-get-import-errors-from-libraries-depending-on-c-modules
-autodoc_mock_imports = ["ESMF", "esmpy"]
+autodoc_mock_imports = ["ESMF"]
 autodoc_member_order = "bysource"
 autodoc_default_options = {
     "members": True,