Drop Python 2.7 and <3.6 support?

[rnestler]

In https://github.com/m-lundberg/simple-pid/commit/990019811041005c6dee23e495f330e3e96d7a0a CI was changed to only test with Python > 3.6.

So would it make sense to drop support for Python 2.7 and Python < 3.5 completely? This would allow / need the following things:

• Get rid of the workaround at https://github.com/m-lundberg/simple-pid/blob/31c5f76f8c71618c69f013506356ee5809fa0ac1/simple_pid/pid.py#L16-L20
• Use inline type annotations instead of the .pyi file
• Remove py2 and pypy from tox.ini
• Remove advertising of py2 support in https://github.com/m-lundberg/simple-pid/blob/master/setup.py#L24