qapi: static typing conversion, pt5a

This is part five, and focuses on QAPISchemaParser in parser.py. It does not touch QAPIDoc yet, which will be covered next.

Requirements:

• Python 3.6+
• mypy >= 0.770
• pylint >= 2.6.0 (2.7.0+ when using Python 3.9+)

Every commit should pass with:

• isort -c qapi/
• flake8 qapi/
• pylint --rcfile=qapi/pylintrc qapi/
• mypy --config-file=qapi/mypy.ini qapi/