IPv6 support

## Driver version

2.1.2

## Redshift version

PostgreSQL 8.0.2 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.4.2 20041017 (Red Hat 3.4.2-6.fc3), Redshift 1.0.70634

## Client Operating System

macos

## Python version

3.9

## Problem description

At my company we started using VPN that only works over IPv6.
Since that we started seeing below errors when connecting:

```
08:57:16 Encountered an error: Database Error ('communication error', gaierror(8, 'nodename nor servname provided, or not known'))
```

I looked into the code and it seems that the driver only works over ipv4.
I started a draft PR: https://github.com/aws/amazon-redshift-python-driver/pull/229
But I'm not sure whether that's enough.

Other SQL clients works fine over IPv6 (datagrip)

首先检查连接代码和 PR #229 草案，然后在 macOS 上使用 Python 3.9，通过仅支持 IPv6 的 VPN 进行连接时重现报告的 gaierror。将驱动程序的行为与报告中可正常工作的 SQL 客户端进行比较，并验证连接能够通过 IPv6 成功建立，同时不破坏对 IPv4 的支持。

由索引模型根据 Issue 内容生成。