chore: run mypy in CI

[agoose77]

This PR enables mypy checks on a subset of the codebase and runs it as part of CI.
Right now, we don't publicly declare type-hints via py.typed, so these are only used internally.

Some things don't type super well; the Sentinel singleton doesn't seem very easy to handle. The closest we could get I think is to use e.g.

import enum
from typing import Literal


class Unset(enum.Enum):
    TOKEN = enum.auto()


def func_with_defaults(x: int, y: int | Literal[Unset.TOKEN]):
    ...

But for now, we'll just ignore this part of the typing.

We're not running in strict mode, so this only checks places where we declare types.

[codecov]

Codecov Report

Merging #2789 (65fc760) into main (c316114) will decrease coverage by 0.05%.
The diff coverage is 75.62%.

Additional details and impacted files

Files	Coverage Δ
src/awkward/_backends/backend.py	88.09% <100.00%> (+0.59%)	⬆️
src/awkward/_backends/cupy.py	66.66% <100.00%> (ø)
src/awkward/_backends/dispatch.py	92.85% <100.00%> (ø)
src/awkward/_backends/jax.py	100.00% <100.00%> (ø)
src/awkward/_backends/numpy.py	100.00% <100.00%> (ø)
src/awkward/_backends/typetracer.py	78.12% <100.00%> (ø)
src/awkward/_kernels.py	66.95% <100.00%> (ø)
src/awkward/_nplikes/dispatch.py	96.00% <100.00%> (ø)
src/awkward/_nplikes/numpy.py	94.44% <100.00%> (ø)
src/awkward/_typing.py	92.85% <100.00%> (ø)
... and 23 more

[agoose77]

65fc760 fixes an existing bug, but a new assertion caught it.