Support bare hostname in proxy

[j178]

Closes #1082

• Auto prepend http scheme for bare hostname in proxy
• Fix two internal use of deprcating .getlist()

[lovelydinosaur]

Fantastic, loving this, thanks! 😃

One interesting question here...
I think we probably only want to support this case wrt. bare environment variables, rather than supporting it in Proxy itself. (?)

Eg. If get_environement_proxies was changed from...

for scheme in ("http", "https", "all"):
    if proxy_info.get(scheme):
        mounts[scheme] = proxy_info[scheme]

to instead be this...

for scheme in ("http", "https", "all"):
    if proxy_info.get(scheme):
        hostname = proxy_info[scheme]
        mounts[scheme] = hostname if "://" in hostname else f"http://{hostname}"

Then we wouldn't change the code in Proxy at all, and our programmatic usage would remain a bit neater and more strict.

client = httpx.Client(proxies="http://localhost:1234")  # Okay
client = httpx.Client(proxies="localhost:1234")         # Invalid, raises an error

While still handling the fuzzy ALL_PROXY=localhost:1234 case gracefully.

Thoughts?

[florimondmanca]

Great stuff! Agreed with ^. :-) Also, if we go the "env vars only" path, we can probably update the Environment Variables: HTTP_PROXY, HTTPS_PROXY, ALL_PROXY docs as part of this PR? (Could be a follow-up task too, don't mind.)

[lovelydinosaur]

Great catch on the get_list/getlist - I've dropped those changes out of this PR.

We can treat them separately (Because I'd probably prefer that we pragma: nocover the to-be-deprecated lines, rather than include them in the test cases like that.)

[lovelydinosaur]

@florimondmanca Yes, good call. General note: We're also missing NO_PROXY there.

[florimondmanca]

Yup. I'm working on a revamp of proxy docs anyway so I can include that stuff there as well.