support typing.Optional in data types conversion

[ophiry]

I'm trying to convert a pydantic model to a schema object
this works fine, even for many complex model defintions.
However, typing.Optional isn't supported.
having support for this would be very useful (even if the schema will ignore the optional aspect, and just treat it as the underlying type)

ibis.dtype(typing.Optional[str]) fails with the error: TypeError: issubclass() arg 1 must be a class