drop support for Python 2.7, 3.5, and pypy

Python 2.7 will reach end of life on Jan 1, 2020. All existing versions of gdbgui will still work and be installable for Python 2.7, but future version will not.

The benefits are:
* Type hints can be added to gdbgui's source code
* asyncio and other standard library code can be used rather than gevent
* use of f-strings
* use of type annotations (Python3.5 doesn't support type annotations on variables)