Fixup intersphinx_mapping for Python and add RTD config file; Fix typo along the way

datalad_deprecated/metadata/metadata.py

@@ -670,7 +670,7 @@ def _get_metadata(ds, types, global_meta=None, content_meta=None, paths=None):
             # important: we want to have a stable order regarding
             # the unique values (a list). we cannot guarantee the
             # same order of discovery, hence even when not using a
-            # set above we would still need sorting. the callenge
+            # set above we would still need sorting. the challenge
             # is that any value can be an arbitrarily complex nested
             # beast
             # we also want to have each unique value set always come

docs/source/conf.py

@@ -115,7 +115,7 @@
 todo_include_todos = True
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'https://docs.python.org/': None}
+intersphinx_mapping = {'python': ('https://docs.python.org/3', None)}
 
 # -- Options for HTML output ----------------------------------------------
 

readthedocs.yml

@@ -0,0 +1,26 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+
+# Set the version of Python and other tools you might need
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.9"
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+   configuration: docs/source/conf.py
+
+
+formats: all
+
+# Optionally declare the Python requirements required to build your docs
+python:
+  install:
+    - path: .
+      method: pip
+    - requirements: requirements-devel.txt