Support OpenAPI 3.1

OpenAPI 3.1 has just been released... Can we hope for support in this library?

Changes in the spec seems really small, but there are some small BCs that could complicate the stuff further: https://www.openapis.org/blog/2021/02/16/migrating-from-openapi-3-0-to-3-1-0

Main differences:
* no more `nullable`
```diff
-type: string
-nullable: true
+type:
+ - "string"
+ - "null"
```
* `exclusiveMinimum` and `exclusiveMaximum` are used directly
```diff
-minimum: 7
-exclusiveMinimum: true
+exclusiveMinimum: 7
```
* `examples`, no more `example`
```diff
-example: fedora
+examples:
+ - fedora
```
* Describing File Upload Payloads
* declaring JSON Schema dialect explicitly

首先查看 issue 中链接的 OpenAPI 3.0 到 3.1 迁移说明，并将其与该库当前的 OpenAPI 处理进行比较。当该库支持所列出的 3.1 变更（包括 nullable 类型、直接排他边界、示例、文件上传 payload 以及显式 JSON Schema 方言）时，即视为完成。

由索引模型根据 Issue 内容生成。