CI: Update Python versions

Avocado has recently dropped Python 3.7, among other reasons because
it has been EOL'ed.

Now, Avocado-VT's CI is broken because it tries to install the latest
Avocado from source and fails.  There are a number of possibilies
here, including limiting the Avocado version for the Python 3.7
checks, but, given that Python 3.7 has been EOL'ed, the most logical
action is to also remove it from Avocado-VT's CI checks.

While at it, let's also expand the coverage with Python 3.11.  Python
3.12 is not so straightforward because of the removal of distutils.

Reference: https://devguide.python.org/versions/
Signed-off-by: Cleber Rosa <[email protected]>

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9, 3.10.1]
+        python-version: [3.8, 3.9, 3.10.1, 3.11]
       fail-fast: false
 
     steps:
@@ -81,7 +81,7 @@ jobs:
 
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9, 3.10.1]
+        python-version: [3.8, 3.9, 3.10.1, 3.11]
       fail-fast: false
 
     steps: