Handle union and literal typing correctly in annotations

[NeejWeej]

This fixes a few issues all related to handling Union and Literals in type annotations. Namely:

fixes #475

Previously, the AST parsing for type annotations was incorrectly converting "None" in the example above to be a TypeVar, and then when we called exec on the new transformed AST, an error was being thrown. We make sure that in the AST parsing, we only convert a string to a TypeVar.

fixes #481

We also handle the case with Literal, where before if you had Literal["a", "b"], "a" and "b" were incorrectly assumed to be CspTypeVar, when they should be the actual literal value. For adjusting the annotations, we stop now if we encounter a Literal annotation.

fixes #482

Related to the Literal and Union typing, when from_dict on csp.Struct was called, we were skipping handling Literal and Union. We update the type normalization code to not treat Literal as a generic container. In from_dict, a Union has no validation performed on it, since under the hood the type normalizer returns object. For Literal, we perform (as we were before in adjusting the annotations themselves) a very basic type check. Namely, we just make sure that we pick a suitable parent class for all the values of Literal (as the type normalization does), and just check it is that class. The tests added demonstrate this, that we don't actually check for Literal when calling from_dict, but just make sure the types line up. So, if Literal[1, 2] exists, then a string passed for that field would error. The nuances of this behavior are addressed in this specific test case: https://github.com/Point72/csp/pull/478/files#diff-16e10f816ba9638999d038fb8bd076b4021e901b60e548b1bc392b6191026687R3907