Datadog

* [BUGFIX] Revert [Return Rate Limit Headers][378], [401][]

* [BUGFIX] Properly extend response headers to response object to fix [Return Rate Limit Headers][378], [397][]

* [BUGFIX] Lambda wrapper: Always flush, even on exceptions, [359][] (thanks [jmehnle][])
* [BUGFIX] API: Do not send JSON body in GET requests, [382][]
* [BUGFIX] API: Allow listing timeboards with empty descriptions, [385][] (thanks [Tenzer][])
* [BUGFIX] Dogwrap: Better string handling and python3 support, [379][]
* [BUGFIX] Threadstats: ensure MetricsAggregator is threadsafe, [370][] (thanks [TheKevJames][])
* [BUGFIX] Dogshell: Fixes the `--tags` argument for service_checks, [387][] (thanks [gordlea][])
* [FEATURE] API: Add support for dashboard list API v2, [374][]
* [IMPROVEMENT] API: Handle http code 429 rate limiting in external python library, [376][]
* [IMPROVEMENT] API: Add ability to not attach_host_name to metrics, events and distributions, [383][]
* [IMPROVEMENT] API: Return Rate Limit Headers, [378][] (thanks [fdhoff][])
* [IMPROVEMENT] API: Do not override API parameters with default when calling initialize if they are already set, [386][]
* [IMPROVEMENT] Dogshell: Add `--tags` support to monitors, [356][]
* [IMPROVEMENT] Dogshell: Add documentation for environment variables, [388][] (thanks [sc68cal][])
* [IMPROVEMENT] Dogstatsd: Added a new parameter `statsd_default_namespace` to the `initialize` method, [353][] (thanks [lceS2][])
* [IMPROVEMENT] Import Iterable from collections.abc on python3 to avoid deprecation warning, [381][]
* [IMPROVEMENT] Do not capture `/bin/hostname` stderr, [368][] (thanks [brendanlong][])
* [IMPROVEMENT] Add support for environment variables `DD_API_KEY` and `DD_APP_KEY` for setting API and APP keys respectively, [373][]

=========

* [BUGFIX] Dogshell: Properly require `handle` as an argument to the `comment` subcommand, [364][]
* [FEATURE] API: Add support for the `Dashboard.get_all` API, [362][]
* [FEATURE] Dogshell: Add support for defining monitors as JSON files, [322][] (thanks [Hefeweizen][])
* [FEATURE] DogStatsD: Add support for the `DD_AGENT_HOST`, `DD_DOGSTATSD_PORT`, and `DD_ENTITY_ID` environment variables, [363][]
* [IMPROVEMENT] API: Add support for the `free` layout_type in `Dashboard.create` and `Dashboard.update`, [362][]

**New Dashboards API: https://docs.datadoghq.com/api/?lang=python#dashboards**

The Timeboard and Screenboard API resources are deprecated in favor of the new Dashboard resource. See https://docs.datadoghq.com/api/?lang=python#dashboards for more details.

* [BUGFIX] API: Fix `UnicodeError` exceptions raised by the API client on errors that contain non ascii characters, [223][], [346][]
* [BUGFIX] DogStatsD: Fix unsafe socket creation on multithreaded applications and catch more exceptions, [212][], [349][]
* [FEATURE] API: Add support for the new Dashboard API, [351][]
* [OTHER] Support `tox` for testing, [342][]
* [OTHER] Support Python 3.7, **drop support for Python 3.3**, [345][]

* [IMPROVEMENT] API: Keep HTTP connections alive when using `requests`, [328][]