fix: what bandit and the remaining lint rules found

- WS-Management answers are parsed with defusedxml (new dependency,
  defusedxml>=0.7) with DTDs forbidden. The check for "<!DOCTYPE" in the raw
  bytes it replaces missed a DTD in a UTF-16 answer, which expat reads too;
  on plain HTTP (16992) anyone in the path can send one. An answer with a
  DTD in any encoding is now refused as no WS-Management answer, as a UTF-8
  one already was (B314, B405).
- xml.sax.saxutils only escapes the request envelopes, nothing parses with
  SAX (nosec B406).
- python -m napalm_amt.record fetches the landing page without certificate
  checks, as before: it sends no credentials and records what discovery
  sees, and AMT's certificate is self-signed unless one was provisioned
  (nosec B501). The driver itself still verifies by default (verify_ssl).

pyproject.toml

@@ -14,6 +14,7 @@ dependencies = [
     # Not on PyPI: install from git before this package (see README).
     "napalm-device-types>=4.16",
     "requests>=2.31",
+    "defusedxml>=0.7",
 ]
 classifiers = [
     "License :: OSI Approved :: Apache Software License",