Use sys_platform instead of platform_system

This is more compatible as a marker and so will break fewer people. Potentially resolves #4006.

I should note that this fix does not entirely remove the issue: sufficiently old pips will still fail to install Requests. We're just moving the goalposts on what counts as "sufficiently old".

I'm in favor of breaking less people's builds as long as people using really old `pip` versions are the ones getting broken still. This change will still carry the benefit of getting users of very old `pip` versions to upgrade, and I actually think that's a positive.

> sufficiently old pips will still fail to install Requests. We're just moving the goalposts on what counts as "sufficiently old".

@Lukasa are you aware of what the specific pip version requirement would be if this got merged?

# [Codecov](https://codecov.io/gh/kennethreitz/requests/pull/4007?src=pr&el=h1) Report
> Merging [#4007](https://codecov.io/gh/kennethreitz/requests/pull/4007?src=pr&el=desc) into [master](https://codecov.io/gh/kennethreitz/requests/commit/508d47dc6e7458f2db3bc3b3bf629e0bdb1b798a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/kennethreitz/requests/pull/4007/graphs/tree.svg?width=650&src=pr&token=mZGswIiXLa&height=150)](https://codecov.io/gh/kennethreitz/requests/pull/4007?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4007   +/-   ##
=======================================
  Coverage   89.74%   89.74%           
=======================================
  Files          15       15           
  Lines        1940     1940           
=======================================
  Hits         1741     1741           
  Misses        199      199
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/kennethreitz/requests/pull/4007?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/kennethreitz/requests/pull/4007?src=pr&el=footer). Last update [508d47d...800a074](https://codecov.io/gh/kennethreitz/requests/pull/4007?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@thieman I'm not. I think it actually boils down to a setuptools requirement, but setuptools from as early as 2014 should handle this I think.

My proposal is that we push a 2.14.1 with this fix, and then see whether things calm down on #4006.

Just to let know I tried with pip 8.0.0 to 8.1.1, and it works.
I also tried with pip 6.0.0, and it works too.
For the cases where the dependency is on setuptools version, I could not test different version so far, sorry.

Still broken on Travis https://travis-ci.org/stripe/stripe-python/jobs/230553497#L498

@ofek thanks for pointing this out. It looks like (some or all of) the Travis-CI instances are running on `setuptools` 12.0.5. While `sys_platform` was supported then, `python_version` doesn't seem to have been added until 18.0 (although the change log says 18.5). You'll need to upgrade  `setuptools` on Travis-CI to at least 18.0, or run `pip install -U setuptools` to get the latest version.