refactor: refactor unnecessary else / elif when if block has a return statement

The use of `else` or `elif` becomes redundant and can be dropped if the last statement under the leading `if` / `elif` block is a `return` statement.
In the case of an `elif` after `return`, it can be written as a separate `if` block.
For `else` blocks after `return`, the statements can be shifted out of `else`. Please refer to the examples below for reference.

Refactoring the code this way can improve code-readability and make it easier to maintain.

src/iSponsorBlockTV/conditional_ttl_cache.py

@@ -41,13 +41,11 @@ class AsyncConditionalTTL:
         def __contains__(self, key):
             if key not in self.keys():
                 return False
-            else:
-                key_expiration = super().__getitem__(key)[1]
-                if key_expiration and key_expiration < datetime.datetime.now():
-                    del self[key]
-                    return False
-                else:
-                    return True
+            key_expiration = super().__getitem__(key)[1]
+            if key_expiration and key_expiration < datetime.datetime.now():
+                del self[key]
+                return False
+            return True
 
         def __getitem__(self, key):
             value = super().__getitem__(key)[0]