Package version number

Currently, the package version number is fetched with [git describe](https://github.com/InsightSoftwareConsortium/ITKPythonPackage/blob/master/scripts/update_python_version.py#L41), but this means that it uses the last tag, which may not be the current tag for new major releases. As an example, the current Python packages are names 4.13.xxxx when the current ITK_VERSION is 5.0.0 which is confusing. A better way to name the packages would be to name them based on the ITK_VERSION variable in the ITK CMakeLists.txt . Following [PEP440](https://www.python.org/dev/peps/pep-0440/#pre-releases), the name of the nightly packages could be: ${ITK_VERSION_MAJOR}.${ITK_VERSION_MINOR}.${ITK_VERSION_PATCH}a${sha}, or something similar.

The current version logic is in scripts/update_python_version.py at the git describe reference; compare it with the ITK_VERSION variable in the ITK CMakeLists.txt. Verify that generated nightly package names use a PEP 440-compatible version based on the current ITK_VERSION rather than an outdated tag.

索引モデルが issue の本文から書いたものです。