Remove support for python 2.7 and 3.4

We had a long conversation in
#matrix-python-sdk:matrix.org
a couple weeks ago about the benefit of only supporting versions of python that
support asyncio and came to the conclusion that unless there are major projects
using python 2 or 3.4 that need this sdk, it would be beneficial to the project
to move to being 3.5+ only.

I'd like it if everyone who has a project using this sdk could comment here
briefly saying whether a switch to 3.5+ only would negatively affect their
project or not. It would be great if you could comment regardless of whether it
would or would not affect you.

从 issue #262 中的讨论以及链接的 #matrix-python-sdk:matrix.org 对话开始；报告中没有指定源文件或测试。确定是否仍有项目依赖 Python 2.7 或 3.4，然后检查仓库的版本支持配置和兼容性引用。支持策略达成一致，并且受影响的项目元数据、代码和测试都一致地面向 Python 3.5+ 时，即视为完成。