Released v15.0

* Add ``Version().is_postrelease`` and ``LegacyVersion().is_postrelease`` to
  make it easy to determine if a release is a post release.

* Add ``Version().base_version`` and ``LegacyVersion().base_version`` to make
  it easy to get the public version without any pre or post release markers.

* Support the update to PEP 440 which removed the implied ``!=V.*`` when using
  either ``>V`` or ``<V`` and which instead special cased the handling of
  pre-releases, post-releases, and local versions when using ``>V`` or ``<V``.