Bump tornado dependency to >=5.0,<6 version

[kozako2]

Description

Bump of tornado version because there is a dependency mismatch when installing this in jupyter/minimal-notebook:d4cbf2f80a2a docker image.

Have you tested this? If so, how?

Tested using tox -e py37. Nevertheless more tests needed,

[dlstadther]

We attempted to update Tornado once before and ran into issues (#2490). Not sure if you were aware. Happy to chat about your problems and proposed solutions!

[daritter]

Hi all,

After looking in #2490, Would it be possible to loosen the tornado requirements if the python version is >=2.7.9 or >=3.4?

# Tornado>5 requires updated ssl module so we only allow it for recent enough versions of python: 3.4+ and 2.7.9+
if sys.version_info[:2] >= (3, 4) or (sys.version_info[:2] == (2, 7) and sys.version_info[2]>=9):
     install_requires.append('tornado>=4.0,<6')
else:
     install_requires.append('tornado>=4.0,<5')

(I would not mind to not care about 2.7 and just relax the requirement for 3.4+).

I'd be happy to make a pr if requested ...

[arc12]

+1 for resolving this; it makes it impossible for me to get a mutually compatible environment.

[honnix]

I believe this is covered by #2761. Please feel free to reopen if this PR intends to cover more.