[Biopython] Switching to PEP440 compliant versioning for our development code

[Peter Cock]

Dear Biopythoneers,

Currently we use a simple two level versioning scheme
of 1.N for Biopython (currently at 1.67), which is all fine for
the recent PEP standards which are important for when
we release code via PyPI, and determining dependencies
and ordering releases.

However, as currently documented on our release process,
in between releases we append a plus, currently 1.67+,
and these versions are not PEP440 compliant.

http://biopython.org/wiki/Building_a_release

The closest equivalent in the standard to 1.67+ (which we
are using to mean after 1.67 but before 1.68) would be
a post release, e.g. 1.67.post

https://www.python.org/dev/peps/pep-0440/#post-releases
https://www.python.org/dev/peps/pep-0345/#version-specifiers

However, having read PEP440, I think it would be clearer
to use 1.78.dev (a development pre-release of what will
be 1.68) instead?

This still allows for alpha, beta and release candidates
should we need them, e.g. 1.78.dev, 1.78a, 1.78b, 1.78rc,
1.78 etc.

i.e. Rather than currently using 1.67, 1.67+, 1.68, 1.68+, ...
I am proposing we use 1.67, 1.68.dev, 1.68, 1.69.dev, ...

Note that pushing development releases to PyPI is frowned
on, and I am not advocating we do that. This should only
be of interest for people who have installed from our git
development repository.

Peter