cephadm: add `types-PyYAML` dependency in mypy testing

cephadm: add `types-PyYAML` dependency in mypy testing

In order to address the following error:

```
cephadmlib/node_proxy/util.py:2: error: Library stubs not installed for "yaml" (or incompatible with Python 3.9)
cephadmlib/node_proxy/util.py:2: note: Hint: "python3 -m pip install types-PyYAML"
cephadmlib/node_proxy/util.py:2: note: (or run "mypy --install-types" to install all missing stub packages)
cephadmlib/node_proxy/util.py:2: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports
```

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>