Remove over-broad Final annotations from overridable constants and instance attributes

[Copilot]

PR #235 marked too many things Final, blocking legitimate monkey-patching of module-level constants (notably HEADER_FORMAT, which is explicitly documented as safe to override) and reassignment of instance attributes like Mesh.name and Mesh.data.

No longer Final

• stl/stl.py: AUTOMATIC, ASCII, BINARY, BUFFER_SIZE, HEADER_SIZE, COUNT_SIZE, MAX_COUNT, HEADER_FORMAT — removed unused Final import entirely
• stl/base.py: AREA_SIZE_THRESHOLD, VECTORS, DIMENSIONS, X, Y, Z, Mesh.name, Mesh.data

Still Final

• Mesh.speedups — invariant after construction; mutating it mid-lifecycle is a bug
• All stl/__about__.py metadata — package identity constants, never modified at runtime

Example — now valid without # type: ignore

import stl

# Explicitly documented as safe to monkey-patch
stl.HEADER_FORMAT = '{name} exported on {now}'

mesh = stl.mesh.Mesh(data)
mesh.name = b'my-solid'  # reassigning instance attribute

[wolph]

Wonderful, so you completely ignored my instructions... very useful