BUG: Fix errors / warnings related to pandas 1.3.0

[maread99]

Fixes #40

[gerrymanoim]

Huh - from scanning the pandas changelog it isn't entirely clear to me what caused this change, but given all the user facing issues I'm going to:

1. Merge this (I've manually confirmed that the behavior is the same is between versions).
2. Cut a release
3. Bump the CI to actually use new pandas/numpy
4. See if dependabot can keep requirements_locked.txt updated more frequently. I want to keep the old compatibilities locked to test backwards compatibility, but we should definitely be triggering CI on new versions of core dependencies.

Thanks for taking this @maread99 - much appreciated!