Remove 'simplejson' as a required dependency

[crccheck]

Hi. I just ran into the same problem outlined in this issue: psf/requests#3052

I've recently run into an issue where another library installed simple-json and because the requests library defaults to it if available, it caused all of our json requests to fail due to a decoding problem.

My opinionated summary of that thread is that in 2018, there's no reason to use simplejson anymore because:

• If people cared about performance, they'd use ujson
• Python versions that didn't have json in the standard lib have long fallen off support grids
• simple is better than complex

Since datadog already has logic to fall back to the standard lib's json, it could simply remove simplejson from setup.py (probably for Python 2 too) and document that simplejson will be used if it's installed.

https://github.com/DataDog/datadogpy/blob/master/setup.py#L12

What do you think? Removing simplejson could simplify this project

[brianblocker]

We are also running into an issue with simplejson as a dependency. Please remove!

[marcus-crane]

Seconding this. We just had some JSON tests that started breaking as requests automatically pulled in simplejson from datadogpy. We'll be replacing the datadog client with a simple HTTP POST in the meantime.

[kapilt]

+1, simplejson hasn't really been needed for a number of years, its been in the stdlib since 2.7, and 2.6 has been out of even security fix support for at least 3 years. https://snarky.ca/stop-using-python-2-6/.. if you still want to reporting rhel6 users, then do it only if the stdlib import fails.

[yannmh]

Closed with #304.