Move _check_mask into core/mask.py and drop dead numpy 1.21/1.22 guards - #607

Two cleanups that share matrix.py and vector.py in disjoint regions.

_check_mask lived in base.py, so mask.py had to defer-import it inside
three methods (Mask.new, Mask.and, Mask.or) to dodge the import
cycle. It is mask logic, so it now lives in mask.py and those three
calls are direct. base.py, matrix.py, and vector.py import it from
mask.py; no new cycle appears, because mask.py has no module-level
import of base. expr.py still needs a function-level import (the import
cycle closes through mask.py's own module-level import of graphblas.binary
names), now pointed at mask.py.

Matrix.from_dicts and Vector.from_dict each carried a np.version
fallback for subarray dtypes on numpy 1.21 and 1.22, where np.fromiter
could not build them. pyproject.toml requires numpy >=1.24, so neither
branch was reachable. The np.fromiter path stays.

What this does not change: same function, same signature, same error
messages. The wrong-kind-mask TypeError and its assignment carve-out
move across intact. The suite count under the pinned config (--backend
suitesparse --blocking --no-mapnumpy) is identical before and
after (1073 passed, 141 skipped).

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