fix: remove extra blank lines between exception classes

After removing pass statements in a previous cleanup, trailing blank
lines inside class bodies created 3 blank lines between classes
instead of PEP 8's standard 2.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoineleclair

src/dqliteclient/exceptions.py

@@ -5,22 +5,18 @@ class DqliteError(Exception):
     """Base exception for dqlite client errors."""
 
 
-
 class DqliteConnectionError(DqliteError):
     """Error establishing or maintaining connection."""
 
 
-
 class ProtocolError(DqliteError):
     """Protocol-level error."""
 
 
-
 class ClusterError(DqliteError):
     """Cluster-related error (leader not found, etc)."""
 
 
-
 class OperationalError(DqliteError):
     """Database operation error."""