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)

Start by reviewing the connection code and draft PR #229, then reproduce the reported gaierror while connecting through an IPv6-only VPN on macOS with Python 3.9. Compare the driver's behavior with the reported working SQL clients and verify that the connection succeeds over IPv6 without breaking IPv4 support.

索引モデルが issue の本文から書いたものです。