Automatically give write access to the author on PR merging

[asvetlov]

The repository has no sensitive data, it is a set of demo projects.
Event accidental commit cannot harm much and can be reverted easily.

On another hand, keeping the repo in the good shape is super important.
A few individuals cannot do it well enough as I can see but lowering the bar can attract help from contributors around the world.

Ideally, we can have post-commit hook in GitHub workflow; but even manual step can be good enough if we have a general agreement on this question.

@aio-libs/admins @aio-libs/aiohttp-committers @aio-libs/aiohttp-demos-writers @aio-libs/triagers -- what do you think?