bpo-37323: Suppress DeprecationWarning raised by @asyncio.coroutine (GH-14293)

bpo-37323: Suppress DeprecationWarning raised by @asyncio.coroutine (GH-14293)

When the test is ran with `PYTHONWARNINGS=error` the environment variable is passed to the python interpreter used in `assert_python_ok` where `DeprecationWarning` from  `@asyncio.coroutine` is converted into an error. Ignore the `DeprecationWarning` in `assert_python_ok`.

https://bugs.python.org/issue37323
(cherry picked from commit 186f70905d07b42a9cbf0a29e448f30baf2b53cb)

Co-authored-by: Xtreak <tir.karthi@gmail.com>