Allow to freeze tryton on OS without python

Use python version defined in the setup.py shebang instead of the generic
python.
(grafted from df77918547985584c40d16939e1029eaa506a5f2)

--HG--
branch : 7.0

tryton/setup-freeze.py

@@ -119,7 +119,7 @@ if os.path.exists(ssl_paths.openssl_capath):
         (ssl_paths.openssl_capath, os.path.join('etc', 'ssl', 'certs')))
 
 version = Popen(
-    'python setup.py --version', stdout=PIPE, shell=True, encoding='utf-8'
+    './setup.py --version', stdout=PIPE, shell=True, encoding='utf-8'
     ).stdout.read()
 version = version.strip()