type_validator: Re-allow subclasses

The previous replaced attrs-strict's type validator with our own,
stricter and faster, validator.

However, the strictness can be a burden in other packages;
for example, swh-storage tests rely on it to insert dummy data that raises
exception when accessed, and it would be hard to do while using the exact
expected type.

This commit reverts the strict behavior, but keeps the performance
optimization, by always checking with type equality, but in case type
equality fails (which would raise an error before this commit), it gives
the value a 'second chance', by trying isinstance.

This means that, outside tests, isinstance should not be used at all,
or very rarely.