Build aarch64 wheels

[dstansby]

This PR:

• Changes the cibuildwheel config to explicitly list the architectures to build for
• Adds aarch64 to this list for linux builds
• Restricts the number of wheels built on a PR to 1 per major platform (linux, windows, macOS x86_64, macOS arm64)
• Builds all the wheels on release, or when the "build all wheels" label is added to a PR

This speeds up runs on PRs, but allows the flexibiility of doing a full run of wheel building if needed by adding the label.

Fixes #312, fixes #288, and an updated version of #315

TODO:

• Changes documented in docs/release.rst
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (4784a52) to head (689c3e4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #563   +/-   ##
=======================================
  Coverage   99.91%   99.91%           
=======================================
  Files          59       59           
  Lines        2334     2334           
=======================================
  Hits         2332     2332           
  Misses          2        2