Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-02-18' into staging

QAPI patches patches for 2021-02-18

# gpg: Signature made Thu 18 Feb 2021 18:51:35 GMT
# gpg:                using RSA key 354BC8B3D7EB2A6B68674E5F3870B400EB918653
# gpg:                issuer "armbru@redhat.com"
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>" [full]
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>" [full]
# Primary key fingerprint: 354B C8B3 D7EB 2A6B 6867  4E5F 3870 B400 EB91 8653

* remotes/armbru/tags/pull-qapi-2021-02-18:
  qapi/introspect.py: set _gen_tree's default ifcond argument to ()
  qapi/introspect.py: Type _gen_tree variants as Sequence[str]
  qapi/introspect.py: Update copyright and authors list
  qapi/introspect.py: Add docstrings to _gen_tree and _tree_to_qlit
  qapi/introspect.py: add type hint annotations
  qapi/introspect.py: remove _gen_variants helper
  qapi/introspect.py: improve readability of _tree_to_qlit
  qapi/introspect.py: improve _tree_to_qlit error message
  qapi/introspect.py: create a typed 'Annotated' data strutcure
  qapi/introspect.py: Introduce preliminary tree typing
  qapi/introspect.py: Always define all 'extra' dict keys
  qapi/introspect.py: replace 'extra' dict with 'comment' argument
  qapi/introspect.py: Unify return type of _make_tree()
  qapi/introspect.py: guard against ifcond/comment misuse
  qapi/introspect.py: add _gen_features helper
  qapi/introspect.py: use _make_tree for features nodes
  qapi/introspect.py: assert schema is not None
  qapi: Replace List[str] with Sequence[str] for ifcond

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>