Adding `nodefaults` to the channel list.

Should limit conda memory consumption that's causing RTD to cancel our
API docs build.

requirements-docs.yml

@@ -5,7 +5,10 @@
 # installed to the readthedocs virtualenv separately.
 channels:
 - conda-forge
-- defaults
+# set nodefaults to remove the default channels
+# this is needed to keep memory use below RTD limit due to conda bug:
+# https://github.com/conda/conda/issues/5003
+- nodefaults
 dependencies:
 - chardet>=3.0.4
 - Cython