try classic solver

Author: Tieqiong

.github/workflows/osx64.yml

@@ -40,7 +40,7 @@ jobs:
         run: |
           conda info
           conda install --file requirements/test.txt
-          conda install wxpython diffpy.utils matplotlib-base
+          conda install --solver=classic wxpython diffpy.utils matplotlib-base
           pip install diffpy.pdffit2==1.5.0rc1
           python -m pip install . --no-deps