Simplify rustbuild Rust beta downloading.

[frewsxcv]

No need to shell out to curl or WebClient when we can just use the
urllib2 library built
into Python.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[xen0n]

No the urllib2 module is Python 2-only, considering #36509 just landed a week ago I think we should only use Py3k-compatible modules from now on.

This should be enough without introducing six as an additional dep:

try:
    from urllib2 import urlopen
except ImportError:
    from urllib.request import urlopen

[frewsxcv]

Ah, didn't realize we supported Python 3, great! Will make the change.

[sanxiyn]

Note that pre-2.7.9 Python does not verify SSL certificates, but curl and WebClient do. Maybe we should warn on old versions of Python?

[alexcrichton]

Thanks for the PR! This was originally done in Cargo as well (where much of this is from) but was reverted for a few reasons:

• urlllib doesn't have proxy support, but curl does
• Python 2/3 compat (looks like that's handled here though)
• Spotty support for https in python, including SNI and such

Since this is intended to be "the most robust method of downloading we can find" it may not be super necessary to unify everywhere (it seems to work well today). For me the lack of proxy plus the spotty support for https in python is a bit of a deal breaker :(

[frewsxcv]

Yeah, thought about @sanxiyn's comment this morning and agree this is probably not a desired change. From what I've heard, Python 2.7.9 > n >= 2.7.0 (very unfortunately) still gets used quite a bit in industry, which results in security issues when using urllib2.