Add support for timezone without colon (similar to datetime)

[Olegt0rr]

Description

Some APIs returns timezone without a colon symbol. Like this:

DT_STRING = "2023-08-25T20:32:38.696+0000"

It's not a problem for python's datetime:

datetime.fromisoformat(DT_STRING)
# 2023-08-25 20:32:38.696000+00:00

But it's a problem for msgspec:

class Issue(Struct):
    created_at: datetime

convert({"created_at": DT_STRING}, MyModel)
# msgspec.ValidationError: Invalid RFC3339 encoded datetime - at `$.created_at`