libxcb: fix `python3` reference. (#110410)
libxcb: fix `python3` reference. (#110410)

See #108008.
lemzwerg/homebrew-core,makigumo/homebrew-core,sjackman/homebrew-core,chrisfinazzo/homebrew-core,makigumo/homebrew-core,chrisfinazzo/homebrew-core,nandub/homebrew-core,stgraber/homebrew-core,Homebrew/homebrew-core,adam-moss/homebrew-core,stgraber/homebrew-core,nandub/homebrew-core,filcab/homebrew-core,adam-moss/homebrew-core,sjackman/homebrew-core,lemzwerg/homebrew-core,Homebrew/homebrew-core,filcab/homebrew-core