refact: extract duplicate code

Author: Vincent Vatelot

slackeventsapi/server.py

@@ -52,25 +52,19 @@ def verify_signature(self, timestamp, signature):
         # Python 2.7.6 doesn't support compare_digest
         # It's recommended to use Python 2.7.7+
         # noqa See https://docs.python.org/2/whatsnew/2.7.html#pep-466-network-security-enhancements-for-python-2-7
+        req = str.encode('v0:' + str(timestamp) + ':') + request.get_data()
+        request_hash = 'v0=' + hmac.new(
+            str.encode(self.signing_secret),
+            req, hashlib.sha256
+        ).hexdigest()
+        
         if hasattr(hmac, "compare_digest"):
-            req = str.encode('v0:' + str(timestamp) + ':') + request.get_data()
-            request_hash = 'v0=' + hmac.new(
-                str.encode(self.signing_secret),
-                req, hashlib.sha256
-            ).hexdigest()
             # Compare byte strings for Python 2
             if (sys.version_info[0] == 2):
                 return hmac.compare_digest(bytes(request_hash), bytes(signature))
             else:
                 return hmac.compare_digest(request_hash, signature)
         else:
-            # So, we'll compare the signatures explicitly
-            req = str.encode('v0:' + str(timestamp) + ':') + request.get_data()
-            request_hash = 'v0=' + hmac.new(
-                str.encode(self.signing_secret),
-                req, hashlib.sha256
-            ).hexdigest()
-
             if len(request_hash) != len(signature):
                 return False
             result = 0