------------------
Fixed
~~~~~
- ``Version`` accepted ``pre=None`` and ``post_tag=None``, which
produces an ambiguous version number. This is because an implicit
pre-release number combined with an implicit post-release looks like
a pre-release with a custom separator:
.. code:: python
>>> Version(release=1, pre_tag='a', pre=None, post_tag=None, post=2)
<Version '1a-2'>
>>> Version(release=1, pre_tag='a', pre_sep2='-', pre=2)
<Version '1a-2'>
The first form now raises a ``ValueError``.
- Don’t allow ``post=None`` when ``post_tag=None``. Implicit post
releases cannot have implicit post release numbers.