rust-python3: useless python2 specific import

This python27_sys import prevents building with python3,
it had been previously removed in a5fa9140ce4c, but that
has been since pruned

Differential Revision: https://phab.mercurial-scm.org/D6415

rust/hg-cpython/src/lib.rs

@@ -23,7 +23,6 b''
 extern crate cpython;
 extern crate hg;
 extern crate libc;
-extern crate python27_sys;
 
 pub mod ancestors;
 mod cindex;