Enforce UTF-8 when open() text files (#460)

Summary:
When opening text files without specifying the encoding (i.e. `open(path, "r")` or `open(path, "w")`), python3 will use the preferred locale encoding (`locale.getpreferredencoding()`) so the result is platform dependent and can change from one machine to another.

I believe fairseq should enforce its standard (UTF-8 seems like the best choice to me). This pull request explicity specify UTF-8 encoding when reading text files.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/460

Differential Revision: D13802525

Pulled By: myleott

fbshipit-source-id: 672fd55707ee559ab36d74bc1c24026166ea2367