pytest+ airtest/poco 警告信息

我用 pytest(5.2.1版本) + airtest/poco 运行测试结束，出现了一下警告！

```shell
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/
airtest/aircv/utils.py:51:
DeprecationWarning: The binary mode of fromstring is deprecated, as it behaves surprisingly on unicode inputs. Use frombuffer instead
nparr = np.fromstring(pngstr, np.uint8)
```

```shell
/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/
airtest/core/api.py:35:
DeprecationWarning: The 'warn' method is deprecated, use 'warning' instead
G.LOGGING.warn("Device:%s updated %s -> %s" % (dev.uuid, instance, dev))
```
虽然不影响执行，但还是希望优化一下代码！