Metadata API: rethink the expiry related API

[MVrachev]

Description of issue or feature request:

In a recent discussion (see discussion in #1722) we reevaluated the need for Singed.bump_expiry() as instead of using the API:

delta = relativedelta(years=1)
timestamp.signed.bump_expiration(delta)

one can just do:

timestamp.signed.expires += relativedelta(years=1)

As a side benefit this will allow the user to use dateutil.relativedelta which is not supported by Singed.bump_expiry as of #1726.

[jku]

Some additional detail here

• I think an actual repository would never bump expiry to previous expiry plus delta. They'd want to bump expiry to today plus delta (if you want reasons why, imagine an original expiry period of 365days: a responsible repository would of course resign much earlier than that, let's say at 121 day mark. Now the next expiry is set 609 days forward. In 121 more days the repository resigns again: now the expiry is 853 days into the future...).
• so a potential reason to include a method is to handle today plus delta and then make sure microseconds are not an issue
• since relativedelta may only be needed for annotations, there is an option of "stringified" annotations (and then importing only if we're statically type checking) but let's only do this if it's really necessary

[jku]

#1712 is about handling microseconds with a set_expiry(): I think these two are related...

[jku]

(Edited to change example to a property instead of a method)

One possibility:

• make expiry a property with a setter function that floors microseconds
• remove bump_expiry()
• document a reasonable way to bump expiry:

  # set expiry to seven days from now
  signed.expiry = utcnow() + relativedelta(day=7)

I think I'm fine with this.

[lukpueh]

(Edited to change example to a property instead of a method)

One possibility:

• make expiry a property with a setter function that floors microseconds

Done as per #1712.

• remove bump_expiry()

Agreed!

• document a reasonable way to bump expiry:

  # set expiry to seven days from now
  signed.expiry = utcnow() + relativedelta(day=7)

I think that's fair.

[lukpueh]

I have a PR in the queue. Will publish tomorrow.