Add TP-Link Archer C20 device tracking support

[btomer]

Description:

Added device_tracker support for my TP-Link Archer C20 router.
Tested on firmware 0.9.1 4.0 v0044.0 Build 160427.

Checklist:

• The code change is tested and works locally.

If the code communicates with devices, web services, or third-party tools:

• Local tests with tox run successfully. Your PR cannot be merged unless tests pass
  (There are no new external dependencies and no new files)

[amelchio]

There is a HA rule that device specific protocol information must be handled by an external PyPI library. Even if the TP-Link device tracker is not a great example of that, a PR making it even worse is unlikely to get accepted at this time.

It seems that someone has already created such a library, from the HA code even: https://github.com/ericpignet/pytplinkrouter

So if you can get your change added to pytplinkrouter instead and then modify this device tracker to use the library, that would be a great contribution!

[ericpignet]

Hi all,
Just a bit of context about this library.
I proposed a PR to add support for TP-Link N600 back in November: #10506
But it was rejected, balloob asked all router specific code to be in an external library.
So I created an external library https://github.com/ericpignet/pytplinkrouter/ with is mostly a copy/paste from the TPLink HA component, with support for my device on top. And I'm using a custom version of tplink.py to use it.
I have not proposed any PR for the custom TPLink component using the new library because it was a very early version: it moves the HA terminology/component functions to the library, but it would be much cleaner to have a more neutral library and HA stuff in HA. I thought there was no rush.
However if others want to add support for new models, I'm happy to already propose a PR with current version, and we can still improve later on.

Eric

[amelchio]

If you are willing to maintain your library for the long haul, changing now makes sense to me.

[balloob]

Please provide a PR to use pytplinkrouter. We should not add new classes to this file.