Allow modifying versions

[hauntsaninja]

Fixes #659

Let me know if something like this seems acceptable to you and I'll add tests and documentation.

Some notes:

• My use case was adding a local version to an existing version
• I don't think it would make sense to allow replacing parts of the release, e.g. "major", since I don't really see a use case. Things like v.replace(major=v.major+1) are probably just mistakes since other parts don't get reset.
• This is why I don't allow replacing epoch. One could make an argument that we shouldn't allow replacing release either, thoughts? (E.g. when would it make sense to replace the release but keep the pre?)
• Version.__new__(Version) is a little gross, as is the munging. The munging is designed to line up with the corresponding properties, so v.replace(xyz=v.xyz) always works.

[brettcannon]

The general approach looks okay to me (with Tzu-ping's comments). @pradyunsg ?

FYI I converted this to a draft until there are tests and documentation.

[pradyunsg]

@pradyunsg ?

Same. I'm on board for this approach, assuming tests + docs + existing review comments are addressed. :)

[doc-sheet]

Hello. If it is not intentional to keep props readonly why not just add setters for smooth increment? e.g.

a = Version('1.2.3')
a.minor += 1

[uranusjr]

If it is not intentional to keep props readonly

Where do you get this impression?