Support Tornado>=6

[maresb]

Sorry to duplicate #2805, but I'm forced to uninstall Luigi from my JupyterLab environment since Tornado < 6 is breaking my extensions. Would you reconsider supporting Tornado 6? Thanks!

[honnix]

According to Tornado release notes: Python 2.7 and 3.4 are no longer supported; the minimum supported Python version is 3.5.2.

However Luigi still claims to support Python 3.3 and 3.4. So there are two options: 1) Luigi drops 3.3. and 3.4; 2) have more branches in setup.py. Wdyt @Tarrasch @dlstadther ? Both 3.3. and 3.4 have reached end of life, so it seems fair to drop them (we are not testing either of those anyway in CI).

[dlstadther]

I'm cool to drop mentioned support of 3.3 and 3.4

[honnix]

Cool. I will work on some PRs to follow this up.

[stonebig]

hum, still no hope for tornado>=6 support ? waiting for tornadoweb/tornado#2815 ?

[honnix]

@stonebig Sorry, blame me on this. I will make a few PRs to move this forward.