gremlin-python tornado >6 support for installation alongside jupyterlab 3.0.0

[Shay Nehmad]

When trying to install the latest `jupyterlab` version (3.0.0, [releases page](https://pypi.org/project/jupyterlab/#history)) alongside the latest `gremlin-python` version (3.4.9, [releases page](https://pypi.org/project/gremlinpython/#history)), I get the following error from `poetry`:

```sh

❯ poetry add jupyterlab

Using version ^3.0.0 for jupyterlab

Updating dependencies

Resolving dependencies... (5.3s)

  SolverProblemError

  Because no versions of jupyterlab match >3.0.0,<4.0.0

   and jupyterlab (3.0.0) depends on tornado (>=6.1.0), jupyterlab (>=3.0.0,<4.0.0) requires tornado (>=6.1.0).

  And because gremlinpython (3.4.9) depends on tornado (>=4.4.1,<6.0)

   and no versions of gremlinpython match >3.4.9,<4.0.0, jupyterlab (>=3.0.0,<4.0.0) is incompatible with gremlinpython (>=3.4.9,<4.0.0).

```

For now, we're just downgrading to `jupyterlab` version ^2.0.0 and it works, but is there any specific reason `gremlinpython` won't work with `tornado` version ^6.0.0?

[Stephen Mallette]

I think the problem with going past 5.0 on 3.4.x is that we're still supporting python 2 there. We don't drop python 2 support until 3.5.0 is released. Note that we've already upgraded to 6.0 there. 

There still remain issues with how gremlin-python makes use of Tornado in general. We don't reuse the existing event loop because we start/stop it between requests given the abstraction we have built around tornado itself. There is some discussion of this here:

https://issues.apache.org/jira/browse/TINKERPOP-1886

We really need to restructure gremlin-python better for 3.5.0 and I think the comments in the above issue give a rough idea of where things are headed. Happy to hear any advice on which direction to take going forward. Please feel free to comment on that issue.

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