Update version.

0.2.3.

Update CI reqs.

Remove cython req from pyproject.

Some minor refactoring for code clarity.

Update changelog.

Merge pull request #26 from materialsvirtuallab/pre-commit-ci-update-…

…config

[pre-commit.ci] pre-commit autoupdate

Clean up pumatgen imports so that it works with older version of

pymatgen.

Update reqs.