Cryptodome/SelfTest/Cipher/test_CAST.pyc

Compiled Python bytecode Reference

A .pyc file contains compiled Python bytecode. The Python interpreter creates the file automatically when it imports a module. The bytecode loads faster than the source, because the parse step is not necessary again. The interpreter keeps these files in __pycache__ directories. The format changes between Python versions, and a magic number in the header identifies the version.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.