Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-05-20' into staging

Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-05-20' into staging

QAPI patches patches for 2021-05-20

# gpg: Signature made Thu 20 May 2021 16:10:21 BST
# 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-2021-05-20:
  qapi/parser: add docstrings
  qapi/parser: allow 'ch' variable name
  qapi/parser: Remove superfluous list comprehension
  qapi/parser: add type hint annotations
  qapi/parser: Rework _check_pragma_list_of_str as a TypeGuard
  qapi/parser: Fix token membership tests when token can be None
  qapi: add must_match helper
  qapi/parser: Use @staticmethod where appropriate
  qapi/parser: assert object keys are strings
  qapi/parser: enforce all top-level expressions must be dict in _parse()
  qapi/parser: Assert lexer value is a string
  qapi/parser: factor parsing routine into method
  qapi/source: Remove line number from QAPISourceInfo initializer
  qapi: Add test for nonexistent schema file
  qapi/parser: Don't try to handle file errors

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