python-devicetree: unpin types-PyYAML version

This is essentially a revert of PR #46311 "python-devicetree: CI hotfix",
assuming the original issue has been resolved, either upstream in
types-PyYAML or in Zephyr itself.

Tested with types-PyYAML 6.0.12.2 (current version at PyPI):

  $ python -m mypy --config-file=tox.ini --package=devicetree
  dtlib.py:962: note: By default the bodies of untyped functions [...]
  dtlib.py:964: note: By default the bodies of untyped functions [...]
  dtlib.py:965: note: By default the bodies of untyped functions [...]
  dtlib.py:967: note: By default the bodies of untyped functions [...]
  Success: no issues found in 4 source files

The "notes" above are harmless (use of type hinting to define local
variables while mypy won't "check the bodies of untyped functions").

References:
- python-devicetree tox run fails (issue #46286)
- python-devicetree: CI hotfix (PR #46311)
- python-devicetree: CI hotfix (commit f6a68434)

Signed-off-by: Chris Duf <chris@openmarl.org>