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.