CI: fix/simplify httpbin use by standardizing Python version

Now that we don't use Travis CI any longer (and don't use httpbin on the
other CI providers yet), we can make our life much simpler by just using
Python 3.10 on all platforms instead of trying to find version-specific
workarounds for various python/pip/httpbin incompatibilities.

Closes #22726.

.github/workflows/ci_mac.yml

@@ -201,6 +201,13 @@ jobs:
         make -C tests $wxBUILD_ARGS failtest
         make -k -C tests $wxBUILD_ARGS "CXXFLAGS=$wxMAKEFILE_CXXFLAGS" "LDFLAGS=$wxMAKEFILE_LDFLAGS"
 
+    - name: Setup Python
+      # actions/setup-python broken on self-hosted https://github.com/actions/setup-python/issues/108
+      if: matrix.runner != 'self-hosted'
+      uses: actions/setup-python@v4
+      with:
+        python-version: '3.10'
+
     - name: Testing
       if: matrix.skip_testing != true
       working-directory: tests