Tests failing with Tornado 6.x version due to missing `stack_context`

Tornado removed the stack_context module (see https://github.com/tornadoweb/tornado/commit/45d9f6d0a23f3ca90d23e00e5c1ed40b955b335d ) so current test runs are failing with:

==================================== ERRORS ====================================
____________ ERROR collecting tests/scope_managers/test_tornado.py _____________
tests/scope_managers/test_tornado.py:27: in <module>
    from opentracing.scope_managers.tornado import TornadoScopeManager
opentracing/scope_managers/tornado.py:24: in <module>
    import tornado.stack_context
E   ModuleNotFoundError: No module named 'tornado.stack_context'
===================== 104 passed, 1 error in 0.89 seconds ======================
make: *** [test] Error 1
The command "make test testbed lint" exited with 2.

[yurishkuro]

we’re already testing with tornado >= 6, and all tests are green in master, so I don’t think this is an issue any longer.

[vissssa]

Should remind ’tornado<6,>=4.1‘