Relax constraints on install requirements

[mwatts15]

The install_requires field in setup.cfg contains several constraints on the versions of packages which specify specific versions of each of the packages. Likely, neuronunit would work with other versions of these, or at least enough of the package would work to be useful. For the sake of downstream users of the library it would be good if the package was less strict about its requirements so that dependency resolution (e.g., with pip) doesn't run into conflicts. It's still possible to keep the constraints available in a constraints file.

[rgerkin]

If you contribute a PR with new, more relaxed constraints, I'd be happy to merge it and update the version.