Add support for -dev for PyPy and other interpreters

[jaraco]

In #213 (comment)_, Ivan suggested that instead of providing support for pre-release versions, users should expect to append -dev to every version. Unfortunately, if one attempts to append -dev to a PyPy version, the action fails with the error:

Invalid 'version' property for PyPy. Both Python version and PyPy versions should satisfy SemVer notation. See README for examples and documentation.

This error is confusing because it suggests that -dev isn't allowed for either (C)Python or PyPy, when in fact appending -dev is the recommended workaround to the behavior that emergent versions of Python are invisible without it on CPython.

In any case, this issue is primarily a feature request to bring PyPy (and any future interpreters) to parity with CPython with -dev support so that -dev doesn't have to be specified in each and every version and pollute the version number in the matrix.

[vsafonkin]

Hi @jaraco, we will take a look at this, thanks.

[mattip]

Hi. PyPy dev here. The list of available pypy versions comes from this file which is managed from the PyPy repo. Would you like the -dev version to be an alias for the nightly PyPy builds? That would be pretty easy to add to the versions.json file.