Warning of deprecated Python API

Running under Python 3.10, I see this warning in the logs:

```
/usr/local/lib/python3.10/site-packages/warctools-4.10.0py3.10.egg/hanzo/warctools/record.py:174: FutureWarning: GzipFile was opened for writing, but this will change in future Python releases.  Specify the mode argument for opening it for writing.
```
