Update Python versions in GitHub actions

Drop Python 2.7, as it is no longer available:

    Warning: The support for python 2.7 will be removed on June 19. Related issue: actions/setup-python#672
    Version 2.7 was not found in the local cache
    Error: The version '2.7' with architecture 'x64' was not found for Ubuntu 22.04.
    The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json

Add Python 3.12 as its support was just added.

.github/workflows/test.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest, macos-latest ]
-        py: [ "2.7", "3.7", "3.8", "3.9", "3.10", "3.11" ]
+        py: [ "3.7", "3.8", "3.9", "3.10", "3.11", "3.12" ]
         rust: [ "1.41.1", "stable", "nightly" ]
 
     steps:
@@ -50,7 +50,7 @@ jobs:
     strategy:
       matrix:
         os: [ windows-latest ]
-        py: [ "2.7", "3.7", "3.8", "3.9", "3.10", "3.11"]
+        py: [ "3.7", "3.8", "3.9", "3.10", "3.11", "3.12" ]
         rust: [ "1.41.1", "stable" ]
 
     steps: