bump required python version to 3.6+ (#560)

homeassistant requires now 3.6.1 or newer, so we can bump our requirement.
this will allow us to convert the code-base to use f-strings.

Closes #494

.travis.yml

@@ -1,7 +1,6 @@
 sudo: false
 language: python
 python:
-  - "3.5"
   - "3.6"
 install: pip install tox-travis coveralls
 script: tox

setup.py

@@ -26,15 +26,14 @@ def readme():
         "Development Status :: 5 - Production/Stable",
         "Intended Audience :: Developers",
         "License :: OSI Approved :: GNU General Public License v3 (GPLv3)",
-        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3 :: Only",
     ],
     keywords="xiaomi miio vacuum",
     packages=["miio"],
     include_package_data=True,
-    python_requires=">=3.5",
+    python_requires=">=3.6",
     install_requires=[
         "construct",
         "click>=7",