- Add Python 3.12 support.
- mmCIF output should now be more complete, and should largely comply with
the PDBx and ModelCIF dictionaries.
- The search path for coordinate files (IOData.atom_files_directory)
now supports both traditional 4-character and new-style 12-character PDB IDs;
for example, PDB file 1abc is searched for in the ab subdirectory, and
file PDB_12345678 is searched for in the 67 subdirectory.
