add a version to make this package compatible with buildout's easy_install

Fixes this issue:

    Installing statsmodels 0.13.5
    Caused installation of a distribution:
    statsmodels 0.0.0
    with a different version.


diff '--color=auto' -ru statsmodels-0.13.5.orig/setup.py statsmodels-0.13.5/setup.py
--- statsmodels-0.13.5.orig/setup.py	2022-11-02 01:54:20.000000000 +0900
+++ statsmodels-0.13.5/setup.py	2023-10-26 14:12:35.157708043 +0900
@@ -387,6 +387,7 @@
 
 setup(
     name=DISTNAME,
+    version='0.13.5',
     maintainer=MAINTAINER,
     ext_modules=extensions,
     maintainer_email=MAINTAINER_EMAIL,
