fix failing CI tests

Author: Tieqiong

.github/workflows/tests-on-pr.yml

@@ -36,7 +36,7 @@ jobs:
         run: |
           conda install --file requirements/test.txt
           conda install wxpython diffpy.utils matplotlib-base
-          pip install diffpy.pdffit2==1.4.4rc6
+          pip install diffpy.pdffit2==1.4.4rc4
           python -m pip install . --no-deps
 
       - name: Start Xvfb