Merge pull request matplotlib#23886 from tacaswell/ci_pin_pikepdf

CI: prefer (older) binaries over (newer) sdists

Author: oscargus

requirements/testing/extra.txt

@@ -1,5 +1,6 @@
 # Extra pip requirements for the Python 3.8+ builds
 
+--prefer-binary
 ipykernel
 nbconvert[execute]!=6.0.0,!=6.0.1
 nbformat!=5.0.0,!=5.0.1