Heaven only knows why the CI pipelines are complaining about a PHP 7.3 issue; all references to PHP 7.3 should have been removed from the workflows

Re-introducing 7.3 now, just to try and ensure that all checks pass, so that the 7.3 weirdness won't be a blocker for merging

.github/workflows/main.yml

@@ -16,6 +16,8 @@ jobs:
         include:
           - php-version: 'nightly'
             experimental: true
+          - php-version: '7.3'
+            experimental: true
 
     name: PHP ${{ matrix.php-version }}