feat: support Python 3.13 and drop the upper Python cap (cyvcf2 >=0.32)

[nh13]

The cyvcf2~=0.31.4 pin was the blocker for Python 3.13: cyvcf2 0.31.x shipped no cp313 build, which forced requires-python to cap at <3.13. cyvcf2 0.32+ adds 3.13 support with an unchanged API, so this relaxes the pin to >=0.32 and drops the upper Python cap entirely — requires-python is now >=3.10 (unbounded).

Because the cap is removed (not merely raised to 3.13), the package is forward-compatible with Python 3.14+ as soon as the dependency builds exist — no further permutect change required. On the conda side that is already true (conda-forge ships pytorch/cyvcf2 py314 builds), so the Bioconda recipe will be able to enable 3.14 directly.

Verified on Python 3.13.14: exercised cyvcf2 0.33.0 against permutect's actual usage (encode_variant, overlapping_filters, find_variant_type, and the filter_variants Writer/header/round-trip path) and ran the unit suite — all green.

3.14 is permitted by requires-python but is not dev-tested here: the pixi dev env can't resolve torch 2.10's macOS-arm64 cp314 wheel (a pytorch wheel-tagging detail, unrelated to permutect), so the pixi dev environment stays capped at <3.14.

Follow-up: once released, the Bioconda recipe can drop its cyvcf2 <0.32 and python <3.13 caps and add py314.

[nh13]

@davidbenjamin once this merges, could we cut a new release (e.g. v0.7.2)? It drops the upper Python cap (cyvcf2 >=0.32, requires-python >=3.10), which unblocks Python 3.13. A release also lets the Bioconda recipe pick it up and drop its own python <3.13 / cyvcf2 <0.32 caps. Thanks!