Fix readthedocs builds

Specify pip doc/requirements.txt instead of using anaconda.  Thanks to
David Stansby.

Closes: #628

.readthedocs.yaml

@@ -0,0 +1,23 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+# Required
+version: 2
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+    # conda:
+    #   environment: ci/doc.yml
+
+# We recommend specifying your dependencies to enable reproducible builds:
+# https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
+python:
+  install:
+  - requirements: docs/requirements.txt
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/source/conf.py
+

ci/doc.yml

@@ -2,6 +2,7 @@ name: glymur-docs
 channels:
     - conda-forge
 dependencies:
-    - python=3.7
+    - python=3.12
     - numpydoc>=0.8
     - sphinx_rtd_theme>=0.4.2
+

docs/requirements.txt

@@ -0,0 +1,2 @@
+numpydoc>=0.8
+sphinx_rtd_theme>=0.4.2