borgmatic: add py3-yaml to depends

This is to fix following error:

/ # borgmatic
Traceback (most recent call last):
  File "/usr/bin/borgmatic", line 33, in <module>
    sys.exit(load_entry_point('borgmatic==1.5.10', 'console_scripts', 'borgmatic')())
  File "/usr/bin/borgmatic", line 25, in importlib_load_entry_point
    return next(matches).load()
  File "/usr/lib/python3.8/importlib/metadata.py", line 77, in load
    module = import_module(match.group('module'))
  File "/usr/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/usr/lib/python3.8/site-packages/borgmatic/commands/borgmatic.py", line 10, in <module>
    import pkg_resources
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 3239, in <module>
    def _initialize_master_working_set():
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 3222, in _call_aside
    f(*args, **kwargs)
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 3251, in _initialize_master_working_set
    working_set = WorkingSet._build_master()
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 567, in _build_master
    ws.require(__requires__)
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 884, in require
    needed = self.resolve(parse_requirements(requirements))
  File "/usr/lib/python3.8/site-packages/pkg_resources/__init__.py", line 770, in resolve
    raise DistributionNotFound(req, requirers)
pkg_resources.DistributionNotFound: The 'PyYAML>=3.11' distribution was not found and is required by pykwalify
/ #