fun local 不支持 python redis 库

## 重现方法

template.yml 文件
```
ROSTemplateFormatVersion: '2015-09-01'
Transform: 'Aliyun::Serverless-2018-04-03'
Resources:
redis:
Type: 'Aliyun::Serverless::Service'
Properties:
Description: This is FC service for redis
python:
Type: 'Aliyun::Serverless::Function'
Properties:
Handler: 'index.handler'
Runtime: python3
Timeout: 10
MemorySize: 128
CodeUri: ./
```

requirements.txt
```
redis
```

index.py
```
import os,sys
print(sys.path)
import redis

def handler(event, context):
return 'hello world'
```

执行命令
```
fun install
# 或者
fun build
```

fun local invoke 报错，部署到远端应该一样会报错的

```
FC Invoke Start RequestId: 1ebcb234-1b1e-48b7-b8ad-d1f55cc7e0c0
['/code', '/var/fc/runtime/python3/src', '/usr/local/lib/python36.zip', '/usr/local/lib/python3.6', '/usr/local/lib/python3.6/lib-dynload', '/usr/local/lib/python3.6/site-packages']
2020-02-11T10:26:25.781Z 1ebcb234-1b1e-48b7-b8ad-d1f55cc7e0c0 [ERROR] {'errorMessage': "Unable to import module 'index'", 'errorType': 'ImportError', 'stackTrace': ["ModuleNotFoundError: No module named 'redis'"]}
FC Invoke End RequestId: 1ebcb234-1b1e-48b7-b8ad-d1f55cc7e0c0
{
"errorMessage": "Unable to import module 'index'",
"errorType": "ImportError",
"stackTrace": [
"ModuleNotFoundError: No module named 'redis'"
]
}
```

## 原因分析

redis 安装目录是 .fun/python/lib/python/site-packages/redis


这个目录没有被 PYTHONUSERBASE=/code/.fun/python 所识别出来，如果把后一个 python改成 python3.6 就可以正常运行

改成 .fun/python/lib/python3.6/site-packages/redis 是 OK 的。

## Workaround

追加环境变量 PYTHONPATH: /code/.fun/python/lib/python/site-packages

Start with the template.yml, requirements.txt, and index.py reproduction, then run `fun install` or `fun build` followed by `fun local invoke`. Trace how the installed Redis package is placed under `.fun/python/lib/python/site-packages` and how the local Python runtime sets its import path. Done means the sample imports `redis` successfully without the PYTHONPATH workaround.

Written by the indexing model from the issue text.