Blmath

As of this release, break from the upstream revision history and adopt ordinary
semver.

Identical to `1.2.5-post3`.

- Add blmath.geometry.shapes.create_rectangular_prism, create_cube,
create_triangular_prism, and create_horizontal_plane
- Add blmath.geometry.surface_normals.surface_normal

- Temporarily drop support for Python 3.

- Fix install issue by removing extraneous dependency.