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

[toabctl]

Tornado removed the stack_context module (see tornadoweb/tornado@45d9f6d ) 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]

Tornado 6.x dropped support for Python 2.x altogether (tornadoweb/tornado#2443).

[condorcet]

I think there is no reason to support TornadoScopeManager for Tornado>=6, because we can use AsyncioScopeManager instead. Here is proof of concept of the way how we can support new Tornado #118

[Jamim]

Probably, this ticket can be closed since #121 fixes the issue as far as I can see.

[yurishkuro]

#121 pins `'tornado<6', so not exactly the solution to this issue.

[vissssa]

Should remind ’tornado<6,>=4.1‘

[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.