Add reusable release workflow

[wpbonelli]

GitHub Actions allows defining reusable workflows and referencing them from external repos. To prevent duplication across repos, a standardized Python package release workflow could be defined here and consumed by

• modflow-devtools itself
• modflowapi
• pywatershed
• flopy
• other EC-related Python projects

This will likely not be possible to reuse for MODFLOW-USGS/modflow6, since it is not a Python project and has a more complex release procedure, although the approach is generally similar.

[wpbonelli]

Part of this effort could be a shared version management tool — currently we have very similar version update scripts in many repositories. That is, unless we adopt something like setuptools_scm or versioneer.

Noting also https://github.com/MODFLOW-ORG/cookiecutter-fortran which is an effort to standardize a recipe for Fortran project repositories.

[wpbonelli]

probably overkill, Python releases are usually simple and it's easy to copy the pattern