Re-add dependency on Python 2 for NEO.

NEO does not currently work with Python 3.
Its dependency with Python 2 was removed in
nexedi/slapos@b3ac97ff
This causes erp5-zope2 to fail during installation with the following
message:

```python
Getting distribution for 'zope.testing==3.9.7'.
  error: subprocess-exited-with-error

  × python setup.py egg_info did not run successfully.
  │ exit code: 1
  ╰─> [7 lines of output]
      Traceback (most recent call last):
        File "<string>", line 2, in <module>
        File "<pip-setuptools-caller>", line 34, in <module>
        File "/tmp/pip-req-build-e3n8vnyw/setup.py", line 37
          except ImportError, e:
                            ^
      SyntaxError: invalid syntax
      [end of output]
```

Thus, we re-add the constraint in this commit.

software/neoppod/software-common.cfg

@@ -23,6 +23,9 @@ extends =
     ../../component/ZEO/buildout.cfg
     ../../component/zodbtools/buildout.cfg
 
+# Python2 versions for buildout (keep last)
+    ../../stack/slapos-py2.cfg
+
 parts =
 # keep neoppod first so that ZODB is built correctly,
 # before any other section that would depend on it