Update for compatibility beyond Julia 1.5

# :rocket:

- [ ] Is your feature request essential for your project?

### Describe the workflow you want to enable
Julia 1.6 has been out for about 8 months now, and is up to 1.6.4 for bug fixes. Julia 1.7.0 seems around the corner (they released their third release-candidate version two weeks ago). Anecdotally, we've heard that using Julia 1.6 breaks some things, either via the main language or via third-party package dependencies, although our test coverage is poor so doesn't catch this. See one discussion at: https://github.com/Breakthrough-Energy/REISE.jl/pull/160#discussion_r756449544

We should figure out exactly what's not working in 1.6, and make changes accordingly, ideally maintaining compatibility with 1.5 if possible. If it's not possible or convenient to maintain backward compatibility, we can make a v0.3.x release for supporting Julia 1.6.x and continue maintaining a v0.2.x release as necessary for Julia 1.5.x.

Once Julia 1.7.0 gets released, we should start evaluating what gaps are there, if any. EDIT: Julia 1.7 has been released, and Julia 1.6.x is now the latest 'long-term support' release.