update versions, for conda test, install mongomock via pip

 * after mongomock infrastructure upgrade, python3.8 is minimum.
   since this is only available via pip, install only this package
   via pip in the conda environment setup

.github/workflows/testing.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
         dependencies: ["pip", "conda"]
       fail-fast: false
 
@@ -53,7 +53,8 @@ jobs:
         if: matrix.dependencies == 'conda'
         run: |
           set -vxeo pipefail
-          conda install -y -c conda-forge six mongoquery doct jsonschema mock mongomock pymongo pytest pyyaml requests tornado ujson
+          conda install -y -c conda-forge six mongoquery doct jsonschema mock pymongo pytest pyyaml requests tornado ujson
+          pip install mongomock
 
       - name: Install the package
         run: |