Fix up the ReadTheDocs build

RTD builds were failing due to the Conda process taking too much memory!
I've figured out how to enable RTD builds during pull request
processing, so we should get much better notification of issues from
here on out. By updating the RTD config file and switching to a
mambaforge installer, it looks like we get things working again.

.readthedocs.yaml

@@ -1,18 +1,17 @@
 version: 2
 
 build:
-  image: latest
+  os: ubuntu-22.04
+  tools:
+    python: mambaforge-4.10
+    nodejs: "18"
   apt_packages:
     - libpci-dev
 
-# As of June 2021, We need to set up a conda environment to install
-# a sufficiently new Node.js: https://github.com/readthedocs/readthedocs.org/issues/8249
-
 conda:
   environment: .readthedocs_env.yml
 
 python:
-  version: 3.8
   install:
     - method: pip
       path: .

.readthedocs_env.yml

@@ -5,31 +5,21 @@ channels:
 dependencies:
   - astropy
   - astropy-sphinx-theme
-  - commonmark
   - ipyevents
-  - ipython
+  - ipykernel
   - ipywidgets
-  - jupyterlab
-  - jupyterlab_widgets
-  - krb5
   - lxml
-  - ipykernel
   - matplotlib
   - nbclassic
-  - nodejs
-  - notebook
   - numpydoc
   - pip
   - pyopengl
   - pyqt
   - pyqtwebengine
-  - python=3.9
+  - python =3.10
   - qt
   - qtpy
-  - recommonmark
   - reproject
-  - scipy
   - setuptools
   - sphinx
   - sphinx-automodapi
-  - widgetsnbextension