Support build reproducibility of non-binary dists

[sethmlarson]

We can't guarantee a given build will be reproducible (because users may be including randomized/uncontrollable elements in their source code/build process) but for builds that are capable of being reproducible we should do everything we're able to do to help.

Some ideas we can pursue:

• SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/
• LC_ALL (C, C.UTF-8): https://reproducible-builds.org/docs/locales/
• PYTHONHASHSEED?: https://docs.python.org/2/using/cmdline.html#envvar-PYTHONHASHSEED

To keep this short I'll focus on non-binary dists in this issue, I believe a lot more work will be needed for reproducibility of binary dists.