Open setup.py in binary mode

[saj]

Fixes #263.

[kwlzn]

mind adding some quick test coverage around this?

it'd be great to prove that your case (./pex27 backoff) and @acaire 's case (./pex34 pygments-markdown-lexer) both work with the binary mode change - and keep it that way going forward.

[kwlzn]

given the active regression, will go ahead and merge this - we can circle back to coverage.

thanks for the bug report and PR.