Feat: select correct pre-releases

[tdejager]

Description

We want to select pre-releases if:

• They are specified in a VersionSet like described in PEP440 e.g foo >= 2.0a1.

But we will not select them if:

• The same dependency is used somewhere in the resolution that does not depend on pre-releases. In that case even though another candidate does depend on it we will not use it.

We will also add a global setting that specifies whether we want to allow pre-releases for a specific package, this entails that it might break some packages that do not support this, but you will not really know until you try it.

Execution

Add support for the pre-releases to PypiVersionSet, this means checking the global parameter and change reading of the verision bounds to allow pre-releases as described in PEP440.

[tdejager]

Does not make a lot sense anymore.