CI: pin oldest supported NumPy

Author: henryiii

.github/workflows/sdist.yml

@@ -58,7 +58,16 @@ jobs:
     - name: Install pandas from sdist
       run: |
         conda list
-        python -m pip install dist/*.gz
+        python -m pip install dist/*.gz -v
+
+    - name: Force oldest supported NumPy
+      run: |
+        case "${{matrix.python-version}}" in
+        3.8)
+          pip install numpy==1.17.3 ;;
+        3.9)
+          pip install numpy==1.19.3 ;;
+        esac
 
     - name: Import pandas
       run: |