build: bump mypy to the current version for Pydantic 2 compatibility
mypy 0.981 doesn't seem to know how to parse pydantic 2 files, reporting errors
with Pydantic's own JsonValue definition.
mypy 1.7.0 and 1.8.0 without the pydantic.mypy plugin spuriously report some
missing named parameters that actually do have defaults.
with the pydantic.mypy plugin and the latest mypy version, all is good,
although implicit optionals will now get flagged as errors because of PEP 484.
Added bonus: this mypy configuration runs much faster!
