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.