API: Make _type read-only

[jku]

Also remove _type from Signed constructor arguments: the value is in a
class attribute. This way _type never needs to be validated (except
in the dispatcher in Metadata). There is a double-check in
_common_fields_from_dict() just to be sure.

This makes the API easier to use correctly as the public property is
immutable.

This is an API break as all Signed constructors change -- this could be
avoided but seems like the correct choice.

Signed-off-by: Jussi Kukkonen jkukkonen@vmware.com

---

Fixes #1371

The discussion-worthy aspects of this PR are:

• it's a breaking change: It changes all Signed-derived constructors API (although calling Signed constructors isn't a common thing to do -- it usually happens in from_dict())
• it uses a property to make an attribute read-only: this is not done so far in the API but I believe we should do that more for attributes that should not be changed or that should be changed via some setter function.

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[joshuagl]

This looks reasonable to me. I like removing _type from the constructor and making the property read-only.

Given that this is an in-flux API, I'm not worried about the API break. Let's get this right before we promise something stable.

@woodruffw is not using the constructors directly in the PyPI/warehouse integration, IIRC. (However I think he already has changes to make as that WIP PR is using Metadata.from_json_file(), which has been renamed Metadata.from_file())