The 3.1.2 version of ArcGIS Pro use the 1.26.0 version of pypdf2. According to PyPI this version dates to 5/18/2016. According to PyPi pypdf2 is moving back to being called pypdf as of version 3.1.0.
Please remove pypdf2 from the base set of installed packages with ArcGIS Pro. Replace with the most recent version of pypdf available at pro release.