Simplify API documentation by using doctests (#73)

* Add doctests to SmartBulb

* Add SmartDevice doctests, cleanup README.md

* add doctests for smartplug and smartstrip

* add discover doctests

* Fix bulb mock

* add smartdimmer doctests

* add sphinx-generated docs, cleanup readme a bit

* remove sphinx-click as it does not work with asyncclick

* in preparation for rtd hooking, move doc deps to be separate from dev deps

* pytestmark needs to be applied separately for each and every file, this fixes the tests

* use pathlib for resolving relative paths

* Skip discovery doctest on python3.7

The code is just fine, but some reason the mocking behaves differently between 3.7 and 3.8.
The latter seems to accept a discrete object for asyncio.run where the former expects a coroutine..

docs/source/discover.rst

@@ -0,0 +1,17 @@
+Discovering devices
+===================
+
+.. code-block::
+
+    import asyncio
+    from kasa import Discover
+
+    devices = asyncio.run(Discover.discover())
+    for addr, dev in devices.items():
+        asyncio.run(dev.update())
+        print(f"{addr} >> {dev}")
+
+
+.. autoclass:: kasa.Discover
+    :members:
+    :undoc-members: