Consider loosening dependency versions

[justinaustin]

Is your feature request related to a problem? Please describe.
I'm working on a python package and would like to include allennlp as a dependency. However, this is proving non trivial as allennlp pins many of its dependencies in the setup.py causing a few of these conflicts. It also looks like these pinned dependencies have caused allennlp to have internally inconsistent dependencies in the past.

Describe the solution you'd like
Ideally, all or nearly all dependencies in the install_requires in allennlp's setup.py would have version constraints instead of pins. This is the recommended practice in python to avoid situations like these. If users need deterministic builds with allennlp, they can use allennlp's provided requirements.txt file. Allennlp also has a nice amount of test coverage, so this should increase confidence in loosening the dependency versions.

Loosening the dependency constraints would also make it easier for others to include allennlp as a dependency in their projects.

Additional context
Tools such as poetry and pipenv can help a lot with this type of dependency management and can detect whenever there are dependency conflicts. It takes a lot of load off of the developer, and I've had luck with them on my projects.

Thank you!

[joelgrus]

I can take a crack at this

[schmmd]

This issue also overlaps with #2280.