Merge pull request RustPython#973 from sanxiyn/remove-extern-crate

Remove extern crate unneeded for Rust 2018

Author: coolreader18

vm/src/lib.rs

@@ -20,13 +20,6 @@ extern crate lexical;
 #[macro_use]
 extern crate log;
 // extern crate env_logger;
-extern crate num_bigint;
-extern crate num_complex;
-extern crate num_integer;
-extern crate num_traits;
-extern crate serde;
-extern crate serde_json;
-extern crate statrs;
 
 extern crate rustpython_parser;
 #[macro_use]