Change dependencies in the Conda environment.

We can remove the dependency on sphinxcontrib, since sphinx.ext.napoleon is
already available. This means we can drop conda-forge, which substantially
reduces build memory usage (see: readthedocs/readthedocs.org#6742,
conda/conda#5003, conda/conda#8650, readthedocs/readthedocs.org#6692).

Bump Sphinx to 2.4.0, while we're at it.

python/docs/conf.py

@@ -71,7 +71,7 @@ extensions = [
   "sphinx.ext.autosummary",
   "sphinx.ext.ifconfig",
   "sphinx.ext.intersphinx",
-  "sphinxcontrib.napoleon",
+  "sphinx.ext.napoleon",
 ]
 
 # Add any paths that contain templates here, relative to this directory.

python/docs/environment.yml

@@ -1,12 +1,10 @@
 name: protobuf
 channels:
-  - conda-forge
   - defaults
 dependencies:
-  - libprotobuf=3.11.2
+  - libprotobuf=3.11.4
   - make=4.2.1
   - pip=19.3.1
   - python=3.7.6
-  - sphinx=2.3.1
+  - sphinx=2.4.0
   - sphinx_rtd_theme=0.4.3
-  - sphinxcontrib-napoleon=0.7