Update docs/env.yml (python-graphblas#338)

It was taking too long to create the environment on readthedocs. This PR makes three changes to improve things:

1. Use mamba instead of conda
2. Add `nodefaults` channel (i.e., only use packages from conda-forge)
3. Install from conda-forge instead of pip where possible and up to date

Adding `nodefaults` actually makes solving the environment pretty fast for both conda and mamba. I suspect mamba downloads files faster.

.readthedocs.yml

@@ -5,5 +5,12 @@ version: 2
 sphinx:
   configuration: docs/conf.py
 
+# Try to make build faster with mamba. See:
+# https://docs.readthedocs.io/en/stable/guides/conda.html#making-builds-faster-with-mamba
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "mambaforge-4.10"
+
 conda:
   environment: docs/env.yml

docs/env.yml

@@ -1,22 +1,23 @@
 name: python-graphblas-docs
 channels:
     - conda-forge
+    - nodefaults
 dependencies:
-    - python=3.9
-    - sphinx>=4.2
-    - graphblas>=7.1.0
-    - python-suitesparse-graphblas
+    - python=3.10
+    - pip
+    # python-graphblas dependencies
+    - donfig
+    - numba
+    - python-suitesparse-graphblas>=7.3.2.0
+    - pyyaml
+    # extra dependencies
     - matplotlib
     - networkx
     - pandas
-    - scipy
-    - numba
-    - cffi
-    - donfig
-    - pyyaml
-    - pip
-    - pip:
-        - nbsphinx
-        - numpydoc
-        - sphinx_panels
-        - pydata_sphinx_theme
+    - scipy>=1.7.0
+    # docs dependencies
+    - commonmark  # For RTD
+    - nbsphinx
+    - numpydoc
+    - pydata-sphinx-theme
+    - sphinx-panels