Omit errors for class pattern matches against object

[cdce8p]

Since the class pattern matches any subclass, it can also be used to check whether the matched object has a specific attribute. Mypy should not emit an error for it.

match m:
    case object(foo=_):
        m.foo

Using object for it is recommended in PEP 635 and more prominently in the precursor PEP 622.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[ilevkivskyi]

LG, this is essentially the same as hasattr().