Add SHA-256 and SHA-512 digest auth algorithms

Added SHA-256 and SHA-512 auth algorithms to requests/auth.py   Added test code to test_requests.py

Requires SHA-256 and SHA-512 in the httpbin test framework.

Seems like the digest stuff is 404ing: mind taking a look?

Can do.

Does need the most recent httpbin with SHA-256 and SHA-512 support.

@Lukasa @linuxlizard, the changes were merged in httpbin but I don't believe we cut a release. I think that may be a prerequisite for pytest-httpbin working here.

I pinged Ian but didn't get a response and wasn't sure what SOP was for httpbin releases/deployments.

Should I move my testing to tests/test_lowlevel.py ?   Would be more of a learning curve for me, take a bit longer.

@linuxlizard, I don't believe there's any reason we need to move down to low level tests here. We just need to get your changes from httpbin into pytest-httpbin.

@sigmavirus24, do we have enough in httpbin to warrant a release so we can get these tests working?

Has there been any movement on this? It's a pretty simple fix but it's killing our router testing. Thanks!

@linuxlizard I believe we’re still waiting on an httpbin release for the tests to be able to run. @sigmavirus24 is there anything we’re waiting on to cut 0.6.3?

@nateprewitt I think there's a new httpbin release that's been out for ~1 week.

@sigmavirus24 yep, this looks fixed. I think the blocker is just appveyor now.

Those are some truly bizarre test failures, but I wonder if they'd be solved by rebasing.

They occur at the tip of master too but aren’t consistent failures. I tried to debug last night without success. I’ll take another look on the plane.

Going to close and re-open this to trigger a new merged build with master

# [Codecov](https://codecov.io/gh/requests/requests/pull/4410?src=pr&el=h1) Report
> Merging [#4410](https://codecov.io/gh/requests/requests/pull/4410?src=pr&el=desc) into [master](https://codecov.io/gh/requests/requests/commit/80e2637a8396c930b8e985e933738136e9ee9c35?src=pr&el=desc) will **increase** coverage by `0.32%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/requests/requests/pull/4410/graphs/tree.svg?token=mZGswIiXLa&src=pr&height=150&width=650)](https://codecov.io/gh/requests/requests/pull/4410?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #4410      +/-   ##
==========================================
+ Coverage   66.21%   66.53%   +0.32%     
==========================================
  Files          15       15              
  Lines        1548     1560      +12     
==========================================
+ Hits         1025     1038      +13     
+ Misses        523      522       -1
```


| [Impacted Files](https://codecov.io/gh/requests/requests/pull/4410?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [requests/auth.py](https://codecov.io/gh/requests/requests/pull/4410/diff?src=pr&el=tree#diff-cmVxdWVzdHMvYXV0aC5weQ==) | `68.96% <100%> (+2.91%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/requests/requests/pull/4410?src=pr&el=footer). Last update [80e2637...371ee7c](https://codecov.io/gh/requests/requests/pull/4410?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


Thanks again @linuxlizard!