Remove lxml >=3.8.0 dependency.

[codypiersall]

The version dependency makes it hard to install nbextensions on certain
platforms (e.g. ARM). See Issue #1172.

[codypiersall]

PR attempting to fix #1172.

[juhasch]

Travis is happy, so this looks good to me.

On a side note: I am using berryconda on Arm, so I did not run into this problem. It provides all nice scientific Python packages in a recent version. You get lxml version 4.1.1 there.

[jcb91]

Well, Travis is happy because it installs lxml 4.1.1, so this PR doesn't change that 😆. However, I did test lxml down to 3.4 at least with tests running ok. I thought maybe we should keep major-version dependency for compatibility reasons (so lxml >= 3), but tbh lxml major versions don't appear to indicate backwards-incompatibility, so whatever, I think this is fine. I've added a small alteration to limit lxml imports to when it's actually used, which should hopefully help limit some of the problems people have had with incompatible dependency libs for lxml.

[codypiersall]

@jcb91 Thanks for doing the extra work of installing older versions and checking that everything still works!