chore: spoon-bot should delete old Revapi reports

[monperrus]

spoon-bot should post a Revapi comment only if the computed API change are different from the last post.

Possible implementation, in revapy.py, use the Github API to go over the list of comments to the PR and compare them against the newly computed one.

[monperrus]

@surli would you have a look at this one?

[pvojtechovsky]

How it should behave when
1st commit has a revapi changes
2nd later commit fixes compatibility problems and there is NO revapi change

In such case there will stay old confusing revapi report.

Idea:

1. each CI run should always delete old revapi changes
2. and generate new one if ther is a revapi change - same like now - or add nothing if there is no change.

[surli]

The solution proposed in #1683 is not working