Move from asyncio.Task.current_task() to asyncio.current_task()

[libnoon]

asyncio.Task.current_task() is deprecated, and generates lots of warnings in the pytests.

$ python3.7 -m pytest tests
[...]
platform linux -- Python 3.7.5rc1, pytest-3.10.1, py-1.8.0, pluggy-0.12.0
[...]
tests/scope_managers/test_asyncio.py::AsyncioCompabilityCheck::test_activate
  /home/noon/git/opentracing-python/opentracing/scope_managers/asyncio.py:110: PendingDeprecationWarning: Task.current_task() is deprecated, use asyncio.current_task() instead
    return asyncio.Task.current_task(loop=loop)
  /home/noon/git/opentracing-python/opentracing/scope_managers/asyncio.py:110: PendingDeprecationWarning: Task.current_task() is deprecated, use asyncio.current_task() instead
    return asyncio.Task.current_task(loop=loop)
  /home/noon/git/opentracing-python/opentracing/scope_managers/asyncio.py:110: PendingDeprecationWarning: Task.current_task() is deprecated, use asyncio.current_task() instead
    return asyncio.Task.current_task(loop=loop)
  /home/noon/git/opentracing-python/opentracing/scope_managers/asyncio.py:110: PendingDeprecationWarning: Task.current_task() is deprecated, use asyncio.current_task() instead
    return asyncio.Task.current_task(loop=loop)
  /home/noon/git/opentracing-python/opentracing/scope_managers/asyncio.py:110: PendingDeprecationWarning: Task.current_task() is deprecated, use asyncio.current_task() instead
    return asyncio.Task.current_task(loop=loop)
  /home/noon/git/opentracing-python/opentracing/scope_managers/asyncio.py:110: PendingDeprecationWarning: Task.current_task() is deprecated, use asyncio.current_task() instead
    return asyncio.Task.current_task(loop=loop)
[...]

[libnoon]

This PR fails on Python 3.5 and 3.6. Should we test the Python version and use the correct function call depending on it?