Relax Table schema constructor

[texodus]

This PR adds support for using native Python types as column type specifiers, when creating a Perspective Table from a schema. The available types are based on the types allowed in 2.10.1:

table = perspective.table({
    "integer": int,
    "float": float,
    "string": str,
    "boolean": bool,
    "date": datetime.date,
    "datetime": datetime.datetime,
})

It is worth noting that despite the use of date and datetime python types as schema specifiers for legacy support, 3.x does not support date or datetime values when provided to the table constructor function in a dict or list input type (though you may extend native json parsing to allow this, or use pandas.DataFrame to marshall these to for you, see the 3.x release notes).

This support is a convenience feature for existing integrations with the old API. Unlike the old API, Table::schema and View::schema (and all existing and future APIs that use Perspective visual types) will only return String.

Also fixed, PerspectiveWidget was moved to a sub-module in #2728, but the reference to this class was left in the root __init__.py's __all__ variable. A test for this has been added as well - which may seem a bit ham-fisted, but even perspective's own repo lacked a test-path which included from perspective import * to expose this case beforehand, so ...