for conda test, install mongomock via pip

 * since mongomock release is only up-to-date via pip,
   install only this package via pip in the conda environment setup

.github/workflows/testing.yml

@@ -53,7 +53,8 @@ jobs:
         if: matrix.dependencies == 'conda'
         run: |
           set -vxeo pipefail
-          conda install -y -c conda-forge doct jsonschema mock mongomock pymongo pytest pyyaml requests tornado ujson
+          conda install -y -c conda-forge doct jsonschema mock pymongo pytest pyyaml requests tornado ujson
+          pip install mongomock
 
       - name: Install the package
         run: |