Remove compatibility check for py25

```
************* Module saml2.s_utils
src/saml2/s_utils.py:385:42: E0602: Undefined variable 'sha' (undefined-variable)
```

This is referenced on a branch of a backwards compatibility check for py25.
This branch will not be reached anymore and it is now removed.

Signed-off-by: Ivan Kanakarakis <[email protected]>

Author: c00kiemon5ter

src/saml2/s_utils.py

@@ -379,10 +379,7 @@ def signature(secret, parts):
             part = part.encode('utf-8')
         newparts.append(part)
     parts = newparts
-    if sys.version_info >= (2, 5):
-        csum = hmac.new(secret, digestmod=hashlib.sha1)
-    else:
-        csum = hmac.new(secret, digestmod=sha)
+    csum = hmac.new(secret, digestmod=hashlib.sha1)
 
     for part in parts:
         csum.update(part)