Updated version to 1.23.0rc1 and updated release date.

Also moved info about rebased implementation lower down in readme.

Updated version number to 1.22.5.

.github/workflows/build_wheels.yml: use ubuntu-latest and macos-latest.

This matches other actions, and avoids use of defunct macos-10.15.

Update version numners and dates for release 1.22.2.

Update version numbers and dates for release.

setup.py tests/: Fixed fstrings to work with python-3.7.

Updated version numbers and dates for 1.21.1 release.

tests/test_pixmap.py: fixed incompatibility with older pythons' fstri…

…ngs.

Update version numbers and dates for 1.21.0 release.

Update version numbers and dates for 1.21.0rc2 release.