Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-04-30' into staging

Merge remote-tracking branch 'remotes/armbru/tags/pull-qapi-2021-04-30' into staging

QAPI patches patches for 2021-04-30

# gpg: Signature made Fri 30 Apr 2021 12:42:32 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-2021-04-30: (25 commits)
  qapi/error.py: enable mypy checks
  qapi/error: Add type hints
  qapi/error.py: enable pylint checks
  qapi/error.py: move QAPIParseError to parser.py
  qapi/error: assert QAPISourceInfo is not None
  qapi/error: Make QAPISourceError 'col' parameter optional
  qapi/error: Use Python3-style super()
  qapi/error: Repurpose QAPIError as an abstract base exception class
  qapi/expr: Update authorship and copyright information
  qapi/expr.py: Use tuples instead of lists for static data
  qapi/expr.py: Add docstrings
  qapi/expr: Only explicitly prohibit 'Kind' nor 'List' for type names
  qapi/expr.py: enable pylint checks
  qapi/expr.py: Remove single-letter variable
  qapi/expr.py: Consolidate check_if_str calls in check_if
  qapi/expr.py: add type hint annotations
  qapi/expr.py: Modify check_keys to accept any Collection
  qapi/expr.py: Add casts in a few select cases
  qapi/expr.py: Check type of union and alternate 'data' member
  qapi/expr.py: move string check upwards in check_type
  ...

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