python: refactor CallSpec2
python: refactor CallSpec2

This type is semi-private; not documented but many plugins access it
through `item.callspec`. However, plugins access the public fields and
almost none try to construct or monkeypatch it. So we should be allowed
to clean it up some.

- Convert to attrs, add slots and frozen

- Instead of doing `new = old.copy(); new.setmulti2()`, do `new =
  old.setmulti()`. This is cleaner and faster.

- Remove the `metafunc` attribute. This causes a reference cycle
  (multifunc._calls -> callspec -> multifunc) for no good reason --
  neither pytest itself or plugins access this attribute, so let's not
  keep the Metafunc objects alive past their due.

- Some comments.

I would have also like to make the dicts and lists themselves immutable,
however some plugins mess with those so that should be done separately,
if at all.

RonnyPfannschmidt/pytest,markshao/pytest,pytest-dev/pytest,Akasurde/pytest