util,python: Updated maintainers.py to fix deprecation warning

util,python: Updated maintainers.py to fix deprecation warning

The following warning was being thrown:

```
util/maint/lib/maintainers.py:120: YAMLLoadWarning: calling yaml.load() without Loader=... is deprecated, as the default Loader is unsafe. Please read https://msg.pyyaml.org/load for full details.
```

This is fixed by adding `Loader=yaml.SafeLoader` when calling
`yaml.load()`.

Change-Id: I3b79115379a45409967a8848175658ab3c13bfc7
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38476
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>