Deprecation warning for python3.5

[wgwz]

This is the deprecation warning needed for #1396. I tested that the deprecation warning works as expected locally and it looks good to me:

(venv) wgwz@icegarden ~/code/trio $ python
Python 3.5.9+ (heads/3.5:276eb67c29, Feb 19 2020, 11:04:36)
[GCC 9.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import trio
__main__:1: TrioDeprecationWarning: Support for Python 3.5 is deprecated since Trio 0.14; use Python 3.6+ instead (https://github.com/python-trio/trio/issues/75)                            
>>>

Running the tests locally worked as expected as well and I now see this warning there too (expected):

venv/lib/python3.5/site-packages/py/_path/local.py:701
  /home/wgwz/code/trio/venv/lib/python3.5/site-packages/py/_path/local.py:701: TrioDeprecationWarning: Support for Python 3.5 is deprecated since Trio 0.14; use Python 3.6+ instead (https://github.com/python-trio/trio/issues/75)
    __import__(modname)

-- Docs: https://docs.pytest.org/en/latest/warnings.html

[njsmith]

CI is saying you need to run yapf, but otherwise looks good.

[njsmith]

Thanks!