Fix - Use GITHUB_TOKEN when downloading zipballs

[gnattishness]

What I did

Have _stream_download() also use GITHUB_TOKEN

Related issue: fixes #1054

How I did it

Move GITHUB_TOKEN retrieval into standalone helper function, pass headers as optional arguments to _stream_download() and _get_mix_default_branch()

Looks like this was also not being used in from_brownie_mix().

How to verify it

Can try installing a github brownie package from a private repo (it now succeeds).

Not too clear how to easily/reasonably include this in the test cases - the test would probably have to rely on an external AUTH token and an active internet connection, or some swanky mocking.

Checklist

• I have confirmed that my PR passes all linting checks
• I have included test cases
• I have updated the documentation
• I have added an entry to the changelog

[gnattishness]

This passes the token through to _get_mix_default_branch() but it's perhaps arguable whether this is useful.
You could argue that any brownie mix (at that url) should be public anyway and shouldn't need the access token.

In that case, it would probably be worth removing the "If this issue persists, generate a Github API token and store..." error message from _get_mix_default_branch().

On the other hand, I suppose it might be useful to use a token for custom rate limiting, and it's not exposing any new info.

[gnattishness]

Let me know what you think, and I'll give it a squish and rebase before you accept 🙂

[iamdefinitelyahuman]

Thanks for this!