chore: prepare the 0.3.18 release

[philomath213]

Prepares the release. Packaging only — no library changes.

• Bump tenseal/version.py to 0.3.18
• Add the 0.3.18 entry to CHANGELOG.md
• Correct the wheel count in RELEASING.md from 16 to 20. ci: build and publish manylinux_aarch64 wheels #529 added the aarch64 row after the previous correction

The rehearsal on main (run #10) published 0.3.17.dev10 to TestPyPI with 21 files: 4 wheels for each of manylinux_2_28_x86_64, musllinux_1_2_x86_64, manylinux_2_28_aarch64, macosx_arm64 and win_amd64, plus the sdist. All 26 jobs passed, and the install-back check passed on all three platforms.

The release closes:

• Unable to install on Alpine Linux #526: Alpine Linux, fixed by the musllinux wheels
• ARM64 wheel build #507: ARM64 wheel build, fixed by the aarch64 wheels
• Tenseal Raspberry pi #512: Raspberry Pi 4 is fixed only if the reporter runs a 64-bit operating system.